A method of inductively coupled plasma mass spectrometry (ICP-MS) was established to determinate trace impurity elements in chitosan. After dissolving in 3% citric acid and low concentration of HCl, the elements including Be, B, Na, Mg, P, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Cd, Sn, Sb, Hg and Pb in chitosan were measured with ICP-MS directly. The addition of low concentration of HCl could effectively improve the stability of all elements, especially Hg and Sn. Dynamic reaction cell (DRC) was used to correct the interference of chlorine ion and other mass spectral interference. The matrix matching and the internal standard elements mixed solution (Sc, Y, In and Bi) had compensated the difference of sample transfer efficiency and evaporation rate of organic solvent. Under the optimum condition, the standard curve of each element exhibits linear correlation coefficient was not less than 0.999 7. The detection limit of the 20 elements was in the range of 0.6～19.0 ng·L-1 while the recovery of standard addition was 89.50%～109.00% with RSD 1.17%～4.05%. The practical sample analysis results showed that the content of impurities in food grade chitosan is very low. And the heavy metal content is extremely low and safe to eat. This method is very simple, sensitive and accurate for the determination of 20 impurity elements in the food grade chitosan.