This paper proposed an optimization design method of freeform reflectors for direct solar radiation measurement to solve the key problem that year-round measurement uniformity is difficult to guarantee due to the tracking device used in the process of direct solar radiation measurement and to achieve tracking-free full-season and full-latitude uniform measurement of direct solar radiation. The global variation law of the direct solar radiation angle was analyzed, and the initial structure model of a freeform reflector was built. An evaluation function of the radiative flux uniformity at spatiotemporal scales (RFUS) was constructed, a dichotomous evolution mechanism and a variable length mechanism without a remainder were developed, and an optimization method of freeform reflectors based on the evolutionary strategy was proposed. For the purpose of achieving full-latitude uniform measurement of direct solar radiation around the world, four representative freeform reflectors with chromosome subdivision parameters of 3°, 1°, 0.5°, and 0.25°, respectively, were iteratively optimized and simulated. The results show that the RFUS is the best, 92.36% to be exact, when the chromosome subdivision parameter is 0.25°. Finally, tests were set up to verify the position of the reflection spot center and the RFUS. According to the results, the maximum linear distance of the spot center obtained by this method is about 1.5 mm, and RFUS is 90.34%. The proposed method can be used to achieve full-spatiotemporal uniform measurement of direct solar radiation, and it provides a theoretical basis and technical support for new products and applications in the field of solar radiation measurement.