Several existing laser diffractometric methods of measuring red cell deformability are analysed and their disadvantages are identified. By establishing the mathematical model of the diffraction image of a red cell group, a new method is presented—measurement of the greyscale of diffraction images. With numerous experiments, this method allows us to statistically obtain the deformation indices of both normal and abnormal red cells, as well as the composition of red cells in the blood and the characteristic values of specific red cells. It is demonstrated that abundant information including characteristics of red cells can be obtained with this method. This method is expected to be promising in both blood disease research and clinical application.