The nonlinear parameter of photonic crystal fiber (PCF) with triangular lattice structure is studied by using the empirical relations and the effective-index model of PCF. More simple empirical relations are summed up, the problems of requiring large computational resources and time in full-vector analysis are resolved by using the empirical relations, and the effective index of the fundamental guided mode and space-filling mode in the triangular air-hole lattice are calculated with it. With the effective-index model of PCF, the PCF with triangular lattice structure is replaced by an equivalent conventional fiber, an effective core area of a PCF of simple expressions is obtained, in which the fiber numerical aperture is used as an association point of the two models. Based on the simple formula, the nonlinear parameter of photonic crystal fibers with different structural/material parameters are fully discussed, and several conclusions coincide with the actual material nonlinearity(the known conclusions) are expressed.