Using a modified ABCD-matrix approach accounting for nonlinear refraction in active medium,the ranges of cavity parameters that provide a mode-locking of Yb:KYW-laser in usual z-fold cavity configuration are determined. Taking the cavity parameters that provide a most efficient mode locking and based on fluctuation model,a numerical simulation of laser operation is performed. In the calculations the side-band pump power of 6W at 982 nm is used in 1 cm length KYW crystal with 1 cm×0. 005 cm beam cross section. Calculations show that self-starting operation is possible with these parameters and dispersion compensation allows for bandwidth-limited ultrashort pulse generation. The shortest pulse duration was determined to be about 200 fs with self-starting buid-up time of 130 μs. Such a built-up time is comparable and even shorter than that one for the lasers with semiconductor saturable absorbers. The region of negative dispersion provided by prism pair for a stable ultrashort pulse generation was determined to be (-17000～-42000)fs2.