Quantitative phase imaging (QPI) of biological cells is an important developing technique. In this paper, a derivative method of phase extraction is put out under several typical blood cells' models, which is completed by numerically simulating the process of QPI. Furthermore, the first-order derivative of the phase is introduced to analyze the cell morphology, and a monocyte model is discussed as an example to demonstrate the method. It shows that the first-order derivative of the phase can be used as a tool for the identification of blood cells.