A liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was developed to determine the plasma concentration of progesterone in Beagle dogs, and apply it to the study of the pharmacokinetics of progesterone sustained-release formulation in Beagle dogs. The plasma samples were processed by protein precipitation method and megestrol acetate was used as an internal standard (IS). The quantitation analysis was performed using multiple-reaction monitoring (MRM) mode at the specific ion transitions of m/z 315.2→97.0 for progesterone and m/z 385.2→267.1 for megestrol acetate (IS) under the positive ion condition. Male Beagle dogs were injected intramuscularly with progesterone sustained-release microspheres and the plasma samples were collected at different time points after administration. The relevant pharmacokinetic parameters were calculated by WinNonlin 8.1 software. A good linearity over the range of 0.1-500.0 ng·mL^-1 was yielded by this method. The intra- and inter-day precision (RSD) were all less than 13.25% and the accuracy (RE) was within 8.92%. Stability test showed that progesterone in dog plasma was stable at room temperature for 12 h, up to 60 days at -20 ℃ and after three cycles of freeze-thaw. The recovery of it ranged from 71.43%-77.97%. After intramuscular injection of progesterone sustained-release microspheres in Beagle dogs, t_max was 19.00 ± 25.36 h, C_max was 137.72 ± 11.59 ng·mL^-1, t_1/2 was 83.83 ± 26.43 h. The drug was released continuously in vivo and in a continuous absorption process for many times with good sustained-release effect. The method developed in this study is sensitive, rapid and stable. It is suitable for the determination of progesterone plasma concentration in Beagle dogs, and can be applied to the preclinical pharmacokinetic study of progesterone-related formulations. The animal experiment scheme of this study was approved by the Animal Ethics Committee of the Academy of Military Medical Sciences.