The present disclosure related to an effort-saving crank structure (1) of a bicycle (8), which includes: a crank mechanism (10) having a crank (11), a shaft end gear (12) and a treadle end gear (13) disposed on the crank (11); a transmission mechanism (20) disposed in the crank (11) and having a rotation shaft (21) and a first gear (22) and a second gear (23) connected to the rotation shaft (21), the first gear (22) and the shaft end gear (12) are engaged for transmission, and the second gear (23) and the treadle end gear (13) are engaged for transmission; a rotation arm unit (30) having a first rotation arm (31) and a second rotation arm (32), two ends of the second rotation arm (32) are respectively connected to the first rotation arm (31) and the treadle end gear (13).