Abstract: A method for the determination of tetrahydrocannabinol (THC) and Δ⁹-tetrahydrocannabinic acid (THC-COOH) in urine by liquid chromatography-tandem mass spectrometry with solid phase extraction was proposed. The urine sample (1 mL) was taken, and 1 mL of 1 mol·L^-1 NaOH solution was added. The mixed solution was heated at 60 ℃ for 15 min, and then passed through activated MAX mixed anionic solid phase extraction column. Using 1 mL of 2% (volume fraction) ammonia solution to fix the targets, with 1 mL of 80% (volume fraction) methanol solution for rinsing, and 1 mL of methanol solution containing 5% (volume fraction) formic acid for elution, the collected eluent was determined according to the working conditions of the instrument. Gradient elution was performed using Agilent Poroshell HPH-C₁₈ column (50 mm×4.6 mm, 4 μm) as the stationary phase, and the mixed solutions composed of 5 mmol·L^-1 ammonium solution and methanol with different volume ratios as the mobile phase. The separated THC and THC-COOH were scanned by electrospray ion source with negative ion mode, and detected by multi-reaction monitoring mode. It was shown that linear relationships between the mass concentrations and corresponding peak areas of THC and THC-COOH were kept in definite ranges, with detection limits (3S/N) of 0.003 μg·L^-1 and 0.05 μg·L^-1, respectively. Test for recovery was made on the blank sample by standard addition method, giving results in the range of 81.2%-99.0%, with precisions of intra-day (n=5) and inter-day (n=5) were in the ranges of 1.1%-6.0% and 2.2%-6.4%, respectively. The method was used to analyze one urine sample of a suspected cannabis user, in which THC and THC-COOH were detected, with detection amounts of 5.8 μg·L^-1 and 56.6 μg·L^-1, respectively.