A simple method using resonator critical stable point is applied to measure the thermal lens focal length of a flashlamp-pumped continuous wave (CW) Nd:YAG (Nd:Y3Al5O12) laser. By measuring output power at special critical stable point, the laser rod thermal lens focal length can be calculated. The method can also be used to obtain the thermal lens radial and tangent focal length, fr and fθ, respectively. This method is simple and practical, it is applicable especially for the measurement of thermal focal length of high power laser. Because of using the critical point of the resonator with laser on, this method is closer real case than without laser, the measurement is more accurate. The measuring deviation of the method comparing with probe beam method is within the order of ±10%. It is less than that of the unstable-resonator method that is in the order of ±20%.