Polyspermy in Tegillarca granosa, which leads to failure or abnormal development of early embryos, appears to be directly related to varying sperm–egg ratios. We investigated the correlation between sperm–egg ratios and conditions of embryonic development by setting six strict sperm–egg ratios and subsequently scoring fertilization rate, abnormal rate, rates of trochophore and D-shaped larvae. When sperm–egg ratios rose from 2 × 102:1 to 1 × 103:1, D-shaped larvae rate dropped significantly from 78.8 ± 13.3% to 47.8 ± 14.9% (analysis of variance, P< 0.05), and to 3.6 ± 2.8% in the 2 × 104:1 group. Using fluorescence microscope observation, polyspermy rates rose from 2.6 ± 0.8% to 72.9 ± 4.8% with increases in sperm–egg ratios. Sperm–egg ratios were therefore correlated with polyspermy rate, which seriously affected larval survival. Under the premise of higher fertilization rate, the sperm–egg ratios of ≤2 × 102:1 appeared to be more beneficial for embryonic development of T. granosa.