A simple method using an astigmatic cavity to determine the thermal lens of laser crystals is reported. At certain resonator parameters there is a certain thermal focal length, which meets the requirement of making the beam size equal at the tangential and sagittal planes. Based on the ABCD matrix, the relation between the cavity length and the focal length is obtained. The relation between the cavity length and the pump power can be established by observing the spot pattern. Thus, the thermal lens length can be obtained by a middle parameter, that is, the cavity length.