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The Ho:YAG laser has been the favored lithotripter for treating urinary calculus since shortly after its introduction in the 1990s because it can fragment all calculus compositions and produces less calculus migration (retropulsion) during treatment than the short-pulsed lasers. Although the lamp-pumped Ho:YAG laser has been commercialized for ureteroscopic laser lithotripsy (URS) for almost 25 years, the lamp-pumped laser rod's transient thermal behavior under an out-of-control lamp discharge has not been reported. A safety question arises: how big is the laser output under an out-of-control lamp discharge? The objective of this study is to simulate the transient thermal behavior of the lamp-pumped laser rod. The temperature profile inside the laser rod is transformed into an equivalent thermal lens, and from the time interval of the variation of the thermal lens between the lasing threshold and the maximum value of the stable region, we can estimate the level of the laser output. The simulation tool used for this study is the Ansys Fluent. The transient thermal behavior of the lamp-pumped laser rod under an out-of-control lamp discharge was reported. Optimization of the pumping pulse for the desired laser output pulse is for future study.