Maximum detection range is an important parameter for evaluating the performance of photon ladar. In this paper, the maximum detection range of direct-detection pulse photon ladar which uses Geiger-mode Avalanche Photodiode (GM-APD) array as the detector is investigated. Based on the laser radar equation and the model of the minimum acceptable detection probability, and assuming the primary electrons triggered by the echo photons in the GM-APD pixel obey Poisson distribution, the theoretical model estimating the maximum detection range is proposed. By using the system design parameters, the influence of five main factors on the maximum detection range is investigated. The results show that the stronger emitted pulse energy, lower noise level, front echo position in the range gate, large atmospheric transmission, and high target reflectivity can result in larger maximum detection range. When the minimum acceptable detection probability is selected to 0.9, by using emitted laser pulse with pulse width 5 ns and energy 50 μJ, the system maximum detection range more than 1 km can be achieved. At the same time, it is important to select the minimum acceptable detection probability for producing a high system detection performance.