It is the unique gain and nonlinear attribute of the Cr4+:YAG crystal,the influence to pulse is discussed by the crystal nonlinear effect from the propagation equation in the Cr4+:YAG laser.It is possible that self modelocked without other modelocked device for the crystal nonlinear effect and the pulse frequency chirp by the crystal nonlinear effect,the soliton pulse is generated in the balance between dispersion and nonlinear effect by dispersion compensation in the cavity,it is one of soliton light source.