Fiber optic probe is made up of two single mode fibers,one for launch and the other for detection,mounted on one gradient index (GRIN) micro lens.It can replace the optical path of the conventional dynamic light scattering set- up and accord with spatial coherence condition.However,there is a lack of the optimal design method of structural parameters of optical fiber probe.Based on spatial coherence condition of the dynamic light scattering system,we analyze the structural parameters of the optical fiber probe,describe the method of optimal design and determine the best value range of the distance and angle between two lens of optical fiber probe.Experimental data shows that the optical fiber probe,whose angle between two lens is 19° and space is 1.5 mm,can meet the requirement of the spatial coherence.In such a case,the signal-to-noise ratio of the received signal is highest and intercept of the correlation function is 0.83.The relative error of the average particle diameter,which is obtained by the optimal fitting Cumulants method,is less than 2%.