By studying the angle relationship between the wave vector and the Poynting vector of the electro-magnetic field in isotropic or anisotropic media，the method for determining the dielectric properties and chiral characteristics of photonic crystals by using the dot product between the unit wave vector and the unit group velocity of

　　the electromagnetic field has been given. It is pointed out that the criterion for the critical value of the chiral charac

　　teristic of photonic crystals can be used to solve the problem of multi-beam combination. At last，the above conclu

　　sions are discussed by analytic calculation. The above results have laid a foundation for further enriching the con

　　tents of electrodynamics and other related course.