The size of charge cloud is examined as a function of acceleration voltage, distance between MCP and anode, and MCP gain. The performance of photon counting imaging detector is significantly affected by charge cloud size. In this paper, the influence of charge cloud size on the performance of UV photon-counting imaging systems is investigated. Modulation distortion, "S" distortion and their cause are discussed. Then, Monte Carlo simulations for anode decode with different charge cloud sizes are carried out. And finally, the effects of acceleration voltage, distance between MCP and WSA, and MCP gain on the system performance are tested and some practical solutions of modulation and "S" distortion are suggested.