Plasma mirrors can be effectively used to improve the signal-to-noise ratio (SNR) of high-power ultrashort laser; however, some experiments conducted using plasma mirrors denote that plasma mirrors may cause focal spot deterioration. Herein, we establish a spatiotemporal focusing multistep propagation algorithm based on plasma expansion and diffraction propagation to quantitatively investigate the plasma-mirror-induced focal spot deterioration. Further, the influences of the plasma expansion time as well as the amplitude and spatial frequency of wavefront error on focal spot deterioration are quantitatively analyzed using a beam quality evaluation function. The simulation results reveal that the plasma-mirror-induced far-field focal spot deterioration can be mainly attributed to the non-uniform plasma expansion based on the plasma expansion time and wavefront error, with the plasma expansion time observed to be the dominant factor. Additionally, higher-amplitude and lower-spatial-frequency wavefront errors have a relatively greater influence on the focusing ability. From the perspective of high SNR ultraintense output capability, the spatiotemporal quality requirement of the pulse is introduced to avoid far-field focal spot deterioration when a plasma mirror is used in a high-power ultrashort laser system.