Compared with the traditional laser altimetry technology, the photon-counting laser altimetry technology has the advantages of large data, light weight, and high ranging precision, which is the development trend of laser altimeter technolgoy. In this paper, we establish a mathematical model to study the characteristics of the photon-counting laser altimetry. The performance of the photon-counting laser altimeter is estimated by numerical calculation. The ground object model is established, and the simulation is carried out with Monte Carlo method. A filtering method for the altimetry data and an algorithm for optimizing the elevation information using the remote sensing images are proposed. The results show that the root-mean-square error of the photon-counting laser altimeter is 6.1 cm under the condition of the noonday background with the most intense sun for the typical ground model, and the error after optimization by the algorithm is 2.6 cm.