The invention discloses a method for forcibly pushing seedling by a conjugate cam type transplanting machine and a planting arm. A driving cylindrical gear movably is sleeved with a planting arm driving shaft; a driven cylindrical gear and conjugate cams are both arranged on a driven shaft; the driving cylindrical gear is extended out of a planting arm casing and is toothed with a planet carrier of a separate planting mechanism and cannot rotate relative to the planet carrier; and the planting arm casing is fixedly arranged on the planting arm driving shaft so as to make seedling taking actions. The conjugate cam, a forked swing link and two rollers fixed on the forked swing link form a conjugate cam mechanism. The forked swing link which is a driven part of the conjugate cam mechanism is fixed at a hinge point on the planting arm casing. One end of the forked swing link is in contact with a main cam and an auxiliary cam of the conjugate cams through the two rollers; and the other end of the forked swing link is in contact with a seedling pushing rod through an elongated slot. The cylindrical gear mechanism and the conjugate cam mechanism are adopted to convert the counterrotating movement of the planting arm driving shaft and the driving cylindrical gear into the straight reciprocating movement of the seedling pushing rod and consequently complete the seedling pushing action. The mechanism can be adapted to the operation of the transplanting machine at any rotating speed.