The aim of work is to develop efficient theoretical model enabling analysis and optimization of Q-switched quasi-threelevel lasers. The model consists of two parts: pumping part and Q-switched part, which can be separated in a case of active Q-switching regime. For the pumping of quasi-three-level gain medium the semi-analytical model was developed, enabling the calculations for average occupation of upper laser level for given pump power and pump duration, spatial pump beam profile, length and dopant level of gain medium. Moreover, ground-state-depletion, up-conversion parasitic relaxation and temperature effects were considered in the model. The new approach for optimization of CW regime of quasi-three-level lasers was developed for Q-switched lasers operating with high repetition rates. Moreover, for long pump durations comparable to laser upper level lifetimes, the optimization procedure based on Lagrange multiplier technique was developed. The simple analytical formulae for effective pump duration needed to achieve the quasistationary inversion for given pump power density and up-conversion parameter were derived. The model enables the optimization of gain medium length and absorbance, average pump area and out-coupling losses for wide class of quasithree-level lasers.