A system including underwater green light laser transmitter and receiver is designed. The optical transmission experiment is made in coastal waters of Weihai and a photomultiplier tube is used as a detector. Different waveforms and signal strengths under different distances are measured by using different receiving loads. The results show that a bigger load will get an increased distance. The effective transmission distance is 26 m when the load is 1 MΩ. Data is fitted under different distances, and it is found that the attenuation in coastal waters appears e exponential decay, the coefficiency is 0.5 m-1. It can be deduced that the system will get a better performance in sea with smaller attenuation.