This paper shows theoretically for the first time that the starting and maintaining of the self-mode-locking of a Kerr medium self-mode-locked laser (KMSMLL) can be made more easily by rationally controlling the parameters of the detuned cavity. According to the properties of the KMSMLL such as the Kerr lens mode-locking (KLM) and the dynamic saturable Gaussian gain, using the laser ray propagational matrix, we calculated the KMSMLL numerically and obtained the most suitable detuning angle of the cavity mirror. The theory agrees with some experimental results and gives new and rational explanations.