The invention relates to an optimized embodiment of a unidirectional seedling conveying device used for a rice transplanter. The unidirectional seedling conveying device comprises a bracket fixedly arranged on a rack, a driving shaft sleeve which is rotationally arranged on the bracket, a transferring arm which is fixedly arranged on an outer wall of the driving shaft sleeve, a driving main shaft which is rotationally arranged in the driving shaft sleeve in a single direction, a driving conveying wheel which is relatively fixedly connected with the driving main shaft, a driven conveying shaft which is rotationally arranged on the rack, a driven conveying wheel which is relatively fixedly connected with the driven conveying shaft, and a conveyor belt arranged between the driving conveying wheel and the driven conveying wheel. The unidirectional seedling conveying device further comprises a fixing shaft sleeve which is fixedly arranged relative to the bracket; and the fixing shaft sleeve is arranged at the outer part of the driving main shaft in a manner that the fixing shaft sleeve and the driving main shaft can rotate in the single direction. According to the unidirectional seedling conveying device provided by the invention, the problem of the rice transplanter that insufficient seedlings are conveyed or no seedlings are conveyed, caused by the reverse rotation of a seedling conveying mechanism is avoided.