Optimum design of the optics structure for the femto-second laser aimed at Cr4+∶YAG crystal is investigated by theoretic analysis and numerical calculation. So as to reduce the complexity, a thin focus lens is plugged into the resonator to replace the fold-mirror. The pair of prisms can affect the stability and astigmatism besides the dispersion compensation, so the optics transmission matrix is deduced to eliminate them. The stability diagrams are obtained for continue-wave-operation and mode-lock-operation with diverse parameters by computer programming, and the way for, selecting the work point is described. The un-match factor (F) of the astigmatism compensation is defined, and the area distribution diagram is painted according to it. The excellent astigmatism compensation can be possessed for running laser on the stable area with F<0.05. The match of pump and oscillating laser beams within the cavity can be achieved by changing the position of the focus lens.