Characteristics of light propagation and polarization,collinear phase matching (PM) problems and methods in the principal planes are discussed based on indicatrix equation for SHG in biaxial crystals.8 polarization combinations for all the possible collinear PM SHG in the principal planes,the PM angle formulas and the general expressions of the effective nonlinear (NL) coefficient d eff are given.The results obtained are prety simple and greate to simplify calculatation and optimum design for the PM.In contrast to a Fresnel surface with double sheets,an indicatrix one is simple.The PM methods searching for all the possible PM polarization combinations of 3 WM which have clearly physical picture,are much simple and easy to be understood based on the indicatrix surface.