For free space optical communication (FSO), various stochastic interfering factors make the communication platform oscillate, as a result the original collimated link is broken, and the bit error rate (BER) increases. For the purpose of reducing BER, the method of controlling the optical spot's shape and size on the receiving end is given, also the principle of obtaining a certain optical spot via astigmatism and the design example are presented in this paper. The result indicates that the BER introduced by vibration could be reduced to the minimum while choosing the optical pot parameters on the receiving end via adjusting the transmitting optical antenna. For the situation of dissymmetry oscillation, the optical spot should be ellipse.