An absolute measuring principle and method of bidirectional reflection distribution function （BRDF） is presented. A spectral radiation meter with 3 nm spectral resolution and a three-dimensional rotating system with 001° accuracy are utilized to establish the automatic measuring platform. The spectral BRDF of typical space target’s surface materials （yellow and silver tinfoil） is measured in the range of 400—2500 nm. Measured results show that the scattering angle，which corresponds to the maximum value of BRDF curve，is generally in the specular reflection direction；the other BRDF values change slowly as scattering angle varies，declining from middle to both sides and approximately following the cosine distribution. A BRDF error analysis method is introduced by using the RMS error calculation，which provides a new way for the BRDF measurement system evaluation. The measurement error is 495%. With five-parameter model of BRDF and the simulated annealing （SA） algorithm，the optimum parameters of the model at each wavelength in the measured spectrum are retrieved at intervals of 1 nm. A comparison of the data calculated by the parametric model with the data measured originally verifies the feasibility and reliability of the modeling. The measuring and modeling results can provide reference data for space target detection and identification.