OBJECTIVE To establish a method for simultaneous determination of 7 anti-tumor drugs （irinotecan， capecitabine， paclitaxel， docetaxel， tamoxifen， letrozole and methotrexate） in human plasma and apply it to the clinic. METHODS After precipitating with a methanol-acetonitrile mixture （1∶ 1， V/V） containing 0.1% formic acid， liquid chromatography-tandem mass spectrometry （LC-MS/MS） was used to determine the plasma concentration， using deuterium isotopes of each analyte as internal standards. The chromatography was performed on the Agilent Eclipse Plus C18 column with a gradient elution of water （containing 0.1% formic acid+0.04% 5 mmol/L ammonium formate） as mobile phase A and acetonitrile （containing 0.1% formic acid） as mobile phase B. The flow rate was 0.6 mL/min， and the column temperature was set at 40 ℃ . The sample size was 10 μL， and the analysis lasted for 5.5 min. Electrospray ionization was used in positive and negative ion mode， and multiple reaction monitoring mode was used. The ion pairs used for quantitative analysis were m/z 587.1→167.1 （irinotecan）， m/z 360.1→244.1 （capecitabine）， m/z 876.4→308.0 （paclitaxel）， m/z 830.3→304.2 （docetaxel）， m/z 372.1→129.1 （tamoxifen）， m/z 284.1→242.1 （letrozole）， and m/z 455.0→ 308.0 （methotrexate）. A total of 97 patients with malignant tumors in our hospital were selected to measure the plasma concentrations of 7 anti-tumor drugs using the above method. RESULTS The linear ranges of irinotecan， capecitabine， paclitaxel， docetaxel， tamoxifen， letrozole and methotrexate were 2-1 000 ng/mL （r＝0.994 3）， 20-10 000 ng/mL （r＝0.997 5）， 2-1 000 ng/mL （r＝0.997 9）， 1-500 ng/mL （r＝0.995 8）， 1-500 ng/mL （r＝0.995 2）， 1-500 ng/mL （r＝0.996 4）， 10-5 000 （r＝0.997 7）， respectively. The quantitative lower limits were 2， 20， 2， 1， 1， 1 and 10 ng/mL； RSDs of intra-assay precision were 0.08%-14.86% （n＝6）. RSDs of inter-batch precision were 1.51%-11.55% （n＝3）， and the accuracies were 89.17%-114.93% （n＝6）. The matrix effects ranged from 89.89%-119.74% （n＝6）. RSDs of the stability tests were 1.98%-14.88% （n＝6）. The results of E-mail：hangpengzhou@163.com clinical application showed， the average plasma concentrations of irinotecan， capecitabine， paclitaxel and docetaxel were 704.09， 909.40， 36.45， 150.43 ng/mL， respectively. The values of the coefficient of variation were 25.24%， 62.65%， 122.69%， and 92.27%. CONCLUSIONS The established LC-MS/MS method is simple and rapid， and can be used for the simultaneous determination of 7 commonly used anti-tumor drugs in the plasma of patients with malignancy.