A part of multivariate function differential calculus is a little more complicated than univariate function differential calculus, which is the key content of 20 16 postgraduate mathematics. Every year, there will be a small question and a big question in the postgraduate entrance examination. The main contents of the investigation are: differentiability of multivariate functions, judgment of derivatives, and derivation of implicit functions. Type questions are calculation and proof, and everyone should pay attention to this part of knowledge.