Abstract: Morphine， O6monoacetylmorphine， codeine and acetylcodeine in heroin addicts’ urine samples were analyzed by on-line solid-phase extraction (column switching) liquid chromatography-tandem mass spectrum (LC-MS/MS). The urine samples were directly injected into a extraction column（Oasis-HLB 2.1 mm×20 mm×25 μm）. After a washing step with the extraction mobile phase, the retained compounds were flushed into the analytical column (Xterra MS C₁₈ 2.1 mm×150 mm×3.5 μm) with the analytical mobile phase and morphine-d₃ as internal standard. The positive electricspray ionization mode was applied, multiple reaction monitoring (MRM) mode was used to analyze target compounds. Identification was based on the compound’s retention time and two pairs of precursor-to-product ion transitions. The limit of detections(LODs) are 1 μg/L for morphine、codeine、O⁶-monoacetylmorphine and 0.3 μg/L for acetylcodeine. The method has excellent linearity， and the linear correlation coefficient is greather than 0.999 1. The method shows high sensitivity and selectivity, which is suitable for analysis of morphine，O⁶-monoacetylmorphine， codeine and acetylcodeine in heroin addicts’ urine samples.