To improve the detection performance of the binary integrator detector (M out of N detector or M/N detector for short) when the spatial scattering density is sparse, the M out of N detector based on scattering density (BSD-M/N) is proposed. First of all, the average false alarm probability and the average detection probability are obtained. And then, the relationship between the detection performance of the proposed detector and the second threshold M is analyzed for different spatial scattering density and different clutter power levels. Moreover the expression of calculating the optimum M is given. Finally, the experimental results show that the proposed detector outperforms the M/N detector when the spatial scattering density is sparse.