在考虑了有限的下能级寿命和激光能级与晶格场的相互作用的基础上，对四能级激光的速率方程组进行了求解。结果表明由于这个有限寿命的存在，激光阈值升高，输出与输入之间出现了非线性。与下能级瞬时跃迁的理想情况不同，泵浦功率增大时，四能级激光器的输出趋向于一个极限值，并且，下能级寿命越长，激光器所能获得的最大输出越小。 Based on the limited lifetime of the lower level and the interaction between the laser level and the lattice field, the rate equations of the four-level laser are solved. The results show that due to this finite lifetime, the laser threshold is increased and nonlinearities appear between the output and the input. In contrast to the ideal case of lower level transient transitions, the output of a four-level laser tends toward a limit when pump power increases, and the longer the lower level, the smaller the maximum output the laser can achieve.