Abstract: The analytical expressions for the intensities of the light diffracted by ultrasonic waves are derived by means of the normal mode approach under such a limited situation that the incident light is normal to the acoustic waves and only three orders （n=0, n=+1 and n= -1） appear in the aeousto-optical diffraction spectrum. The results are coincident with that given by Nath based on the Ramen-Nath equations. A criterion for the validity of those expressions is obtained by using numerical calculations implemented in the range of 1 ≤ Q ≤10,and it is demonstrated that the expressions will be valid as long as the condition of s=w/Q≤0.3 maintains.