Due to the influence of absorption and scattering of seawater, the amplitude of received signal reduces and the signal-to-noise ratio (SNR) reduces during long distance underwater communications, which leads to the bit error rate (BER) of underwater laser communication systems rise, and eventually limits the distance of communication. For this reason, the maximum ratio combining (MRC) diversity reception technology applied in the underwater laser communication system is put forward. The improvement of MRC spatial diversity reception technology compared with the equal gain combining (EGC) spatial diversity reception technology is analyzed under the influence of absorption and scattering of water. Weighting coefficient distribution of MRC is deduced. The relationship between receiver number and system BER performance is analyzed. Monte Carlo method is used to simulate the performance improvement of MRC based on 532 nm laser at transmission distance of 100 m in Jerlov IB water with six receivers. In the IB and II type water, the relationship between BER and transmission distance is given for MRC and EGC. The theoretical analysis and simulation results show that MRC can distribute gain coefficients more reasonably according to SNR in each receiving branch, to achieve the optimal SNR. Under the requirement of same BER, MRC can realize the longest transmission distance of underwater laser communication. Under the requirement of same transmission distance, MRC can reduce BER of communication system. An engineering solution for long distance underwater laser communication system is provided.