The allotetraploid in cross-fertilized eggs of Chlamys farreri♀× Patinopecten yessoensis♂ was induced in this paper. In order to find out the appropriate duration time of 6-DMAP treatment, ploidy status of induction offspring was analyzed by flow cytometry. The results showed that the optimal duration time of 6-DMAP treatment was to treat the fertilized eggs with 50 mg/L 6-DMAP for 15min based on the integrated evaluation of the results of fertilization rate, survival rate of D-larvae, development status of larvae, and percentage of tetraploid. The results of flow cytometry showed that 17.3% tetraploid was obtained which showed that allotetraploid could be gotten with this method. The reasons of the problems in the work were also discussed in this paper.