A disposable puncture depth adjustable safety lancet consists of a shell, a lancet core, a spring and a trigger, wherein the shell is fixed by at least two flexible claws and all flexible claws form a flexible clamp; wherein the lancet core is held in the flexible clamp and the spring pushes a tail of the lancet core and acts on the ejecting direction of the lancet core; wherein the flexible clamp uses a hook at an end and clamping force to clamp the lancet core to be in locking state before triggering; wherein a head of the trigger extends out of a front end of the shell; wherein a body of the trigger is slideably connected in the shell; wherein the trigger is equipped with a bevel forcing an end of the flexible clamp to stretch to a side direction in relative to the at least two flexible claws; wherein, under an assembly state, the shell is provided with an adjusting ring to adjust a puncture depth in the front end, and the adjusting ring is a cylindrical-shape component; wherein the adjusting ring is sleeved at an outside of the trigger head and it is clearance fit with the trigger, and an end face of the trigger head to contact a skin is exposed out of a port at a front end of the adjusting ring; wherein the adjusting ring and the shell are connected in a rotational way in a peripheral direction of the lancet core, and the adjusting ring and the shell are connected in a fixed way in an axial direction of the lancet core, and a rotational locating structure is set between the adjusting ring and the shell in a rotation direction; wherein the adjusting ring is provided with an inner end face on an inner wall, and the inner end face faces a rear end direction of the adjusting ring, and the inner end face is a rotating step face or rotating bevel, and the lancet core is provided with a side wing in relative to the rotating step face or rotating bevel, an end face of the side wing facing the rotating step face or rotating bevel is a collision face, and the trigger is provided with a guide slot in relative to the side wing, and the guide slot is provided with an opening at the outside of the trigger head, so that the collision face rushes to the rotating step face or rotating bevel through the opening during an ejection of the lancet core; and wherein a rotation of the adjusting ring locates the adjusting ring in different locating positions at a peripheral direction of the shell, and the rotating step face or rotating bevel meets the collision face of the lancet core in different axial positions, thereby changing the puncture depth of a needle tip. wherein the shell is fixed by at least two flexible claws and all flexible claws form a flexible clamp; wherein the lancet core is held in the flexible clamp and the spring pushes a tail of the lancet core and acts on the ejecting direction of the lancet core; wherein the flexible clamp uses a hook at an end and clamping force to clamp the lancet core to be in locking state before triggering; wherein a head of the trigger extends out of a front end of the shell; wherein a body of the trigger is slideably connected in the shell; wherein the trigger is equipped with a bevel forcing an end of the flexible clamp to stretch to a side direction in relative to the at least two flexible claws; wherein, under an assembly state, the shell is provided with an adjusting ring to adjust a puncture depth in the front end, and the adjusting ring is a cylindrical-shape component; wherein the adjusting ring is sleeved at an outside of the trigger head and it is clearance fit with the trigger, and an end face of the trigger head to contact a skin is exposed out of a port at a front end of the adjusting ring; wherein the adjusting ring and the shell are connected in a rotational way in a peripheral direction of the lancet core, and the adjusting ring and the shell are connected in a fixed way in an axial direction of the lancet core, and a rotational locating structure is set between the adjusting ring and the shell in a rotation direction; wherein the adjusting ring is provided with an inner end face on an inner wall, and the inner end face faces a rear end direction of the adjusting ring, and the inner end face is a rotating step face or rotating bevel, and the lancet core is provided with a side wing in relative to the rotating step face or rotating bevel, an end face of the side wing facing the rotating step face or rotating bevel is a collision face, and the trigger is provided with a guide slot in relative to the side wing, and the guide slot is provided with an opening at the outside of the trigger head, so that the collision face rushes to the rotating step face or rotating bevel through the opening during an ejection of the lancet core; and wherein a rotation of the adjusting ring locates the adjusting ring in different locating positions at a peripheral direction of the shell, and the rotating step face or rotating bevel meets the collision face of the lancet core in different axial positions, thereby changing the puncture depth of a needle tip.