The output characteristics of a LD end-pumped Cr4+∶YAG passively Q-switched Nd:YAG laser are experimentally investigated. The experiment shows that the output power and pulse repetition rate increases with the cavity length. To explain above experiment phenomenon, the pump laser beam sizes in the Nd:YAG crystal are measured, and the fundamental mode laser beam radius in the Nd∶YAG crystal and the Cr4+∶YAG crystal are calculated. The investigations show that two reasons result in the output power and pulse repetition rate increasing with the cavity length. Firstly, the pump beam size in the Nd∶YAG crystal is far less than the fundamental mode laser beam in the Nd:YAG crystal. The overlap efficient between pump and the laser beam is low. With the cavity length increasing, the fundamental mode laser beam in the Nd∶YAG crystal decreases, which results in the overlap efficient between pump beam and the laser beam increasing. So the output power and the pulse repetition rate increase with the cavity length increasing. Secondly, the laser beam radius in the Cr4+∶YAG crystal also decreases with the cavity length, which shorten the time interval between two bleaching of the Cr4+∶YAG crystal. So the pulse repetition rate increases with the cavity length.