A fast,simple method for the determination of nicotine and its metabolites in human saliva was developed and validated by liquid chromatography-tandem mass spectrometry（LC-MS/MS）.Human saliva was collected with Salivette- collection kit and the quantification of analytes was achieved by using nicotinemethyl-d3,cotinine-methyl-d3 and trans-3′-hydroxycotinine-methyl-d3 as internal standard under atmosphere pressure chemical ionization（APCI） mode.The pretreatment of this method was simple and fast,and all analytes were eluted within 1 min.The relative standard deviations（RSDs） for nicotine and its two metabolites were between 0.29% and 4.0% and the recovery ranged from 94% to 104%.The linear correlations were above 0.998.Saliva,plasma and urine samples from 70 volunteers were collected and quantified by the described procedure.The classification of different smoking amount was also made using ada-boost based on the total concentration of salivary nicotine and its metabolites by a predicting accuracy of 76%.A good correlation of cotinine levels bewteen saliva and plasma（r=0.92）,saliva and urine（r=084） was found.The results showed that the cotinine in saliva has a longer half-life and becomes the most desirable biomarker for the evaluation of smoking exposure and nicotine uptake.