A reverse brake mechanism for electrically assisted bicycle, comprising: a motor for outputting an assisted power; a chainwheel comprising a socket coupled to said motor for rotation in one direction and rotatable by said motor in a forward direction; a crankshaft rotatably inserted through said socket of said chainwheel; wherein the reverse brake mechanism is characterized in that the reverse brake mechanism further comprises: a drive ring fixedly mounted around said crankshaft and coupled to said socket of said chainwheel in such a manner that when said crankshaft is rotated forward, said drive ring is driven by said crankshaft to rotate said socket and said chainwheel forward; when said crankshaft is rotated backward, said drive ring is driven by said crankshaft to rotate relative to said socket and then to rotate said socket and said chainwheel backward after a predetermined time lag; two magnetic devices and each comprising a magnet holder and a magnet, said two magnet holders being respectively fixedly mounted at said socket of said chainwheel and said drive ring, the magnet of each said magnetic device being fixedly mounted in the associating said magnet holder in such a manner that when said crankshaft is rotated forward, said two magnet holders are synchronously driven by said socket of said chainwheel and said drive ring to rotate forward; when said crankshaft is rotated backward, one said magnet holder is driven by said drive ring to rotate relative to the other said magnet holder in said predetermined time lag, causing a polar misalignment between the magnet of one said magnetic device and the magnet of the other said magnetic device; and two magnetic sensors electrically connected to said motor and respectively facing toward the magnets of said two magnetic devices and for sensing the polar misalignment between the magnet of one said magnetic device and the magnet of the other said magnetic device and interrupting the output of the assisted power of said motor. a motor for outputting an assisted power; a chainwheel comprising a socket coupled to said motor for rotation in one direction and rotatable by said motor in a forward direction; a crankshaft rotatably inserted through said socket of said chainwheel; wherein the reverse brake mechanism is characterized in that the reverse brake mechanism further comprises: a drive ring fixedly mounted around said crankshaft and coupled to said socket of said chainwheel in such a manner that when said crankshaft is rotated forward, said drive ring is driven by said crankshaft to rotate said socket and said chainwheel forward; when said crankshaft is rotated backward, said drive ring is driven by said crankshaft to rotate relative to said socket and then to rotate said socket and said chainwheel backward after a predetermined time lag; two magnetic devices and each comprising a magnet holder and a magnet, said two magnet holders being respectively fixedly mounted at said socket of said chainwheel and said drive ring, the magnet of each said magnetic device being fixedly mounted in the associating said magnet holder in such a manner that when said crankshaft is rotated forward, said two magnet holders are synchronously driven by said socket of said chainwheel and said drive ring to rotate forward; when said crankshaft is rotated backward, one said magnet holder is driven by said drive ring to rotate relative to the other said magnet holder in said predetermined time lag, causing a polar misalignment between the magnet of one said magnetic device and the magnet of the other said magnetic device; and two magnetic sensors electrically connected to said motor and respectively facing toward the magnets of said two magnetic devices and for sensing the polar misalignment between the magnet of one said magnetic device and the magnet of the other said magnetic device and interrupting the output of the assisted power of said motor. a drive ring fixedly mounted around said crankshaft and coupled to said socket of said chainwheel in such a manner that when said crankshaft is rotated forward, said drive ring is driven by said crankshaft to rotate said socket and said chainwheel forward; when said crankshaft is rotated backward, said drive ring is driven by said crankshaft to rotate relative to said socket and then to rotate said socket and said chainwheel backward after a predetermined time lag; two magnetic devices and each comprising a magnet holder and a magnet, said two magnet holders being respectively fixedly mounted at said socket of said chainwheel and said drive ring, the magnet of each said magnetic device being fixedly mounted in the associating said magnet holder in such a manner that when said crankshaft is rotated forward, said two magnet holders are synchronously driven by said socket of said chainwheel and said drive ring to rotate forward; when said crankshaft is rotated backward, one said magnet holder is driven by said drive ring to rotate relative to the other said magnet holder in said predetermined time lag, causing a polar misalignment between the magnet of one said magnetic device and the magnet of the other said magnetic device; and two magnetic sensors electrically connected to said motor and respectively facing toward the magnets of said two magnetic devices and for sensing the polar misalignment between the magnet of one said magnetic device and the magnet of the other said magnetic device and interrupting the output of the assisted power of said motor.