Composite crystals are very effective for controlling the thermal effect in end-pumped solid-state lasers. We calculate the temperature-field distribution and thermal focal length in both YVO4/Nd:YVO4 and Nd:YVO4 rectangular crystals with the end-pumped arrangement. Since it is impossible to calculate precisely the thermal focal length in composite crystals directly by the equations, this is usually done by analyzing additional optical path differences (OPD) caused by heat. For this, we use the temperature distributions in OPD calculations. The results show that, at a pump power of 25 W, the maximum temperature on the rear facet of an Nd:YVO4 original crystal is 560 K, while for the YVO4/Nd:YVO4 composite crystal it is 430 K, Also under the same conditions, the thermal focal length in the composite crystal is larger than in the original crystal, as expected.