Abstract: A method for the simultaneous determination of 21 plant growth regulators in swelling agent type agricultural fertilizer used for traditional Chinese medicine by high performance liquid chromatography-tandem mass spectrometry was proposed. The agricultural fertilizer sample (0.1 mL) was taken, and 10 mL of 50% (volume fraction) acetonitrile solution was added for extraction. Then 150 mg of MgSO₄ and 50 mg of C₁₈ adsorbents were used for purification, and the internal standard was added into the supernatant. After vortex and filtration, the subsequent filtrate was injected into the high performance liquid chromatograph-tandem mass spectrometer. Gradient elution was performed on Agilent Poroshell 120 EC C₁₈ column with 0.1% (volume fraction) formic acid solution-acetonitrile as mobile phase system. The multiple reaction monitoring mode was used for detection on the positive and negative mode of electrospray ionization source, with internal standard method for quantification. As shown by the results, linear relationships between the mass concentrations of 21 plant growth regulators and the peak area ratios were kept in definite ranges, with detection limits in the range of 0.01-4.00 μg·kg^-1, and RSDs of the determined values less than 5.0%. Test for recovery was made according to the standard addition method, giving results in the range of 71.6%-105%.