An online cleanup/LC-MS/MS method was established using the restricted access molecularly imprinted polymer cartridge against β-agonist after the optimization of the condition of online cleanup and combination of switching valve.The samples were extracted with ammonium acetate buffer after enzyme hydrolysis,and cleaned up with an online molecularly imprinted cartridge.The detection of clenbuterol and ractopamine was carried out by MS/MS under multiple reaction monitoring mode after separated on a CAPCELL PAK C18 column（2.1 mm×100 mm,5 μm）.An internal standard method was used for quantitative analysis.The limits of quantitation for clenbuterol and ractopamine were 0.02 μg/kg and 0.09 μg/kg,respectively.The spiked recoveries for clenbuterol at spiked concentration levels of 0.05-0.20 μg/kg,and for ractopamine at spiked concentration levels of 0.5-2.0 μg/kg in pork,beef,sausage and milk powder samples were between 84.0% and 103.8% with RSDs less than 12%.