We have studied the emission spectrum of CdS single crystal in the excitation range from 10

6

to 10

7

W/cm

2

at temperature 4.2K.When the excitation density is low (106 W/cm

2

)

the emission spectrum is characterized by two peaks 4869Å (named I

2

) and 4888Å (named I

1

) which are the luminescence lines of neutral-donor-bound eXciton and of neutral-acceptor-bound exciton. As the excitation density goes up

the intensity of I

2

-peak decreases relatively and a luminescence band structure appears in the red side of I

1

-peak. When the excitation density up to 1.7×10

7

W/cm

2

a new peak (named U) is found. The new peak is characterized by narrow line width (about 3Å half width) and great luminescence intensity (Fig.1)We studied the polarization of I

1

-peak

I

2

-peak and U-peak as the excitation density changed. We found that the polarization v.s. excitation density for I

1

and U is the same. And we also studied the U-peak luminescence intensity by changing temperature in a fixed excitation. And the result is the same with that of I

2

-peak.Finally we concluded that the U-peak is the inelastic scattering luminescence of neutral-donor-bound exciton.We also caculated the intensity and energy of U-peak theoretically

the results are in correspondence with experimental results.We suggested the model of luminescence of inelastic bound exciton-exciton scattering.The causation and features of the peak were clearly explained using the model.