Influence of energy-transfer upconversion (ETU) effect on the output pulse in diode-pumped Cr:YAG passively Q-switched lasers is investigated. Using the ETU theory and Q-switching rate equations, the initial population inversion, the final population inversion and the threshold population inversion at the maximum photon density are achieved. The output pulse width and pulse energy depend on the relation between them. The predicted results are in good agreement with the experimental results. The method of reducing the ETU effect is also discussed and it is useful to optimize this kind of lasers.