The channel capacity of coherent pulse-position modulation (PPM) and maximization conditions of information transmitting rate were analyzed. After the probability distribution of the PPM signal detected by heterodyne method is analyzed, the transition probabilities matrix and capacity of optical pulse-position modulated channel with coherent receiver and maximum likelihood detection were deduced. It is proved that under the circumstance of shot-noise limit detection, the capacity of the channel is only related to number of received photons and the PPM order. Jensen′s inequality is used to simplify the capacity of the channel and obtain its low limit, which is close to the capacity of the channel of practical communication link. The relation of the optimum order of coherent-PPM corresponding to the dead time for the greatest transmitting rate is estimated. General relationship between the optimum PPM order and the dead time was given. Using the proposed method, the estimated PPM-order can realize the maximum capacity of channel when received photons can promise the symbol error rate lower than 10-2.