Aiming at the necessity of underwater single-beam scanning detection system detecting near field target, optical parameters were optimized to suppress the backscatter by sea and improve system SNR. Blind distance formula, echo power equation of underwater target and sea backscatter were derived to analyze the relationship among detection area, system SNR and optical parameters. The optical parameters were optimizationally calculated based on particle swarm optimization(PSO) under different water qualities. The results showed that, for the system detection area was 6-10 m, when the transmitted beam was parallel to receiving field, half receiving FOV was 9 mrad, and the baseline distance was 10.6 cm, the system got the best SNR. To prove the correctness of the theoretical model, optical simulation system of underwater laser detection was designed to test system SNR under different optical parameters, the experimental result was consistent with theoretical result. The optimization results can provide theory basis for designing optical parameters of underwater single-beam scanning detection system.