There are advantages of small volume and light weight in optical communications. For the smooth communication of long distance, the transmitting power should be calculated with higher security efficiency. The constraints of transmitting power, beam divergence and the detecting sensitivity on the communication distance and performance is analyzed. A calculation equation with higher security efficiency is proposed. When the error rate is determined, the minimum detecting power could be calculated by the relationship with signal-to-noise ratio (SNR). In the real communication system, the minimum detecting power could be tested by the relationship with the detecting sensitivity. The calculated equation is verified through the agreement of above two results. The communication distance demonstration is realized by the calculation equation with higher security efficiency.