A sliding block type stepless autoshift speed changer includes a holder sleeve affixed to a main transmission shaft and holding two weights by means of two torsional members, an eccentric block radially movably coupled to the holder sleeve; a driving wheel supported on a ring round the eccentric block and having radial sliding grooves on two sides, two clutch wheels respectively fastened to a wheel hub and a secondary power output shaft, each clutch wheel having an annular sliding groove respectively facing the driving wheel, sliding blocks respectively sliding in the annular sliding grooves of the clutch wheels, each sliding block having a roller respectively coupled to and movable along the radial sliding grooves of the driving wheel, a plurality of beveled guide surface portions, and a plurality of rolling cylinders rotatably supported on and movable along the beveled guide surface portions.