By simultaneously using both active and passive Q-switched in the same cavity,a diode pumped doubly Q-switched c-cut Nd:GdVO₄ laser was realized with acoustic-optic (AO) modulator and Cr4+:YAG saturable absorber. The technique to control the pulse duration has been studied. With varying the positions of the saturable absorber and the modulator,the pulse durations are varied,respectively. The average output power,the pulse duration and the peak power versus the position were obtained. Using the ABCD matrix method,the intracavity radii along the cavity axis were calculated. Mean while,a rate equation model is introduced to theoretically analyze the results obtained in the experiment,in which the Gaussian spatial distribution of the intracavity photon density and the AO turnoff time including the transit time are taken into account. The numerical calculations of the rate equations,are consistent with the experimental results.