The invention discloses a method for determining residence time of an afterloading source by using a regularization method. The method comprises the following steps of (I) according to a CT (computed tomography) image of a patient, determining a target region and outlines of critical organs, and setting a position of a residence point, a position of a reference point and prescription dosage; (II) calculating the dosage of the reference point according to a formula (1): img file='DDA0000620438600000011.TIF' wi='352' he='192'; (III) constructing an evaluation function (2) according to the deviation between the calculated dosage and the prescription dosage and time difference of adjacent residence positions, wherein the evaluation function (2) is that O(T)=(D0-AT)2+alphaTT.T; (IV) performing differential on a residence time vector T by using the evaluation function to obtain a formula (3): imag file='DDA0000620438600000012.TIF' wi=690' he='136'; (V) simplifying the formula (3) to obtain a regular equation set, and expressing the equation set by the vector: (B+alphaI)T=F; (VI) changing different alpha values to obtain different residence time solution sets. The afterloading residence time is obtained by the regularization method, and a time smoothing factor is introduced to solve the problem of large difference of the residence time at adjacent residence positions.