To study the effect of solution pH value on the chelation structure of zinc acexamate. A series of samples at different solution pH values were prepared by 10% HCl or 1 mol.L-1 NH3.H2O. Then API/TOFMS with electrospray ion source was applied to assay the samples. The nitrogen curtain gas and nebulizer gas were adjusted to a constant flow rate of 0.6 microL.min-1 and 2 microL.min-1 respectively. Samples were infused into the electrospray interface using a 500 microL syringe pump at a flow rate of 5 microL.min-1. Mass spectra were acquired in positive ion modes by scanning over the range of m/z 100-1,000. The chelation structure of zinc acexamate is stable at pH 2.54 and it can be easy to form the ion (M + ZnY)+ (Y = CH3CONH(CH2)5COO-) and (2M + Na)+ in this condition. The drug is an effective antiulcer agent. It may decrease the acidity of stomach juice, and form a polymer to protect the ulcer.