Abstract: Retrieving apparent source time function (ASTF) is an important way to study earthquake source parameters. This paper proposes an improved particle swarm optimization (PSO) algorithm for retrieving ASTF. The result from water-level method is used as the initial global best value of PSO, and the inertia factor and learning factor of PSO are modified to increase computing efficiency. The results of simulation experiments using the improved PSO method are compared with those by using projected Landweber deconvolution (PLD) and generic algorithm (GA). The comparison shows that the inversion result by using improved PSO has smaller residuals relative to that by using PLD, and the improved PSO has increased computation efficiency by more than 500% relative to GA. Finally, the improved PSO method is used to retrieve P-wave ASTFs of the 8 October 2005 Kashmir M_W7.6 earthquake. The results show that the duration of P-wave ASTF of the earthquake is within 25 s, and the rupture extends in northwest direction, which are consistent with those by Zhang et al.