A major water leak prevention apparatus intended to be mounted on a water inlet valve body of a water inlet valve, said water inlet valve being connected to a water tank, the apparatus comprising a swing arm, a regulation bar, a float, a water container, a water receptacle tray, and a counterweight float; the swing arm being intended to be rotatably mounted to the water inlet valve body, the swing arm having an end to which a back pressure pad is mounted to correspond in mounted state to a back pressure hole of the water inlet valve body, the swing arm having an opposite end connected to the regulation bar, the float being mounted on the regulation bar and received in the water container, the water container being intended to be mounted on the water inlet valve body; the water receptacle tray being intended to be mounted on the water inlet valve body and is located above the water container, the water receptacle tray comprising an overflow port formed thereon, the overflow port being aligned with the water container, the water receptacle tray having a bottom in which an opening is formed; and the counterweight float being intended to be mounted on a lower end of the water inlet valve body, the counterweight float comprising a water compartment and a buoyance compartment, the water compartment and the buoyance compartment being arranged in mounted state at the lower end of the water inlet valve body, the bottom opening of the water receptacle tray being connected, via a passageway pipe, to the counterweight float, the major water leak prevention apparatus being characterized in that: the water compartment and the buoyance compartment are independent of each other but integrated together; wherein in a condition where the gravity of the water compartment is greater than its buoyance, the water compartment blocks a lower end opening of the passageway pipe; and in a condition where the gravity of the water compartment is smaller than its buoyance, the water compartment opens the lower end opening of the passageway pipe; and in that the major water leak prevention apparatus further comprises a siphon tube mounted on the water container, the water container not being provided with an opening in a bottom thereof, the siphon tube being configured to drain, substantially completely, water out of the water container when the water tank suffers continuous major water leakage, causing the swing arm to rotate thereby allowing the back pressure pad of the swing arm to open the back pressure hole of the water inlet valve body such that water flows through the water inlet valve body into the water tank, so that the water inlet valve enables water ingress, whereby water enters the water receptacle tray and flows via the opening in the bottom of the water receptacle tray into the passageway pipe, filling the water compartment with water while it looses buoyance due to the major water leakage in the water tank, thereby causing the water compartment to block the lower end opening of the passageway pipe, so that water that flows into the passageway pipe flows backward to pass through the overflow port of the water receptacle tray and enters the water container; and whereby the float received in the water container is caused to ascend, due to the raise of the water level in the water container, and to drive the swing arm to rotate thereby allowing the back pressure pad to close the back pressure hole, whereby water cannot flow through the water inlet valve body into the water tank, so that ingress of water into the water tank is stopped. the water compartment and the buoyance compartment are independent of each other but integrated together; wherein in a condition where the gravity of the water compartment is greater than its buoyance, the water compartment blocks a lower end opening of the passageway pipe; and in a condition where the gravity of the water compartment is smaller than its buoyance, the water compartment opens the lower end opening of the passageway pipe; and in that the major water leak prevention apparatus further comprises a siphon tube mounted on the water container, the water container not being provided with an opening in a bottom thereof, the siphon tube being configured to drain, substantially completely, water out of the water container when the water tank suffers continuous major water leakage, causing the swing arm to rotate thereby allowing the back pressure pad of the swing arm to open the back pressure hole of the water inlet valve body such that water flows through the water inlet valve body into the water tank, so that the water inlet valve enables water ingress, whereby water enters the water receptacle tray and flows via the opening in the bottom of the water receptacle tray into the passageway pipe, filling the water compartment with water while it looses buoyance due to the major water leakage in the water tank, thereby causing the water compartment to block the lower end opening of the passageway pipe, so that water that flows into the passageway pipe flows backward to pass through the overflow port of the water receptacle tray and enters the water container; and whereby the float received in the water container is caused to ascend, due to the raise of the water level in the water container, and to drive the swing arm to rotate thereby allowing the back pressure pad to close the back pressure hole, whereby water cannot flow through the water inlet valve body into the water tank, so that ingress of water into the water tank is stopped.