The thermal effect is one of the factors that must be considered in designing the fiber coupled laser diode (LD) end-pumped solid-state lasers. The most convenient way of studying the thermal effect is using an ideal lens with equivalent thermal focal length. The old expression of the thermal lens (TL) focal length gives values smaller than the actual one. The cause leading to the difference is studied and a new more precise method is given. Numerical calculation is done and curve fitting equations are presented. The results show that the focal length is not only a function of the pump power but also a function of the ratio of the laser radius to the pump radius. When the laser radius is about the same as the pump radius, the ratio of the new value of the focal length to the old one is about 1.6. And at the same condition, the new value of the diffraction loss is much smaller than the one calculated by the old value of the focal length, and is also a function of the pump radius and the laser radius. Experiments show that the new values are closer to the measured results than the old ones.