Effect of annealing on the properties of time-resolved spectra of photoluminescence (PL) and the decay in PL intensity of black silicon processed by femtosecond laser pulses in air is studied. The motion of photon-generated carriers and the property of defects in black silicon after annealing have been analyzed. By fitting the PL decay profile with double exponential function, the probable mechanism of spectral change of black silicon due to annealing is given. The increase in PL intensity of black silicon after annealing is the result of the increased oxygen defects resulting from the radiative recombination rate of nonequilibrium carriers. A theory of thermal diffusion is put forward to explain the observation that the PL intensity of black silicon increases with the annealing temperature. The increasing time constants and the ratio of the slow decay process are attributed to the fact that some defects on the microstructured surface and within silicon material have been eliminated and restored by annealing that leads to the increasing of the surface bound state, thus reducing the density of nonradiative recombination centers. The ratio of radiative recombination increases, resulting in the increase in PL intensity of annealed black silicon. The best annealing condition for the PL of black silicon found is annealing at 800 ℃ for 30 min in vacuum.