Microstructure fiber whose cladding is composed of three air-holes is designed. Its mode characteristics are investigated by full vector finite element method. Because of C3v symmetry, the fiber contains 4 distinct classes of mode and the fundamental mode is degenerate mode. The dispersion and phase mismatch in fundamental mode of the fiber is analyzed when d/Λ=0.986，d changes from 10 μm to 16 μm and d=14 μm，d/Λ changes from 0.946 to 0.986. It is found that with fixed pump, changes on fiber structure parameters have great influence on the position of Stokes signal while the position of anti-Stokes signal stays almost unchanged. Four-wave mixing and frequency conversion experiments are carried out in the outer cladding of a homemade double cladding microstructure fiber. Stokes and anti-Stokes signals at 1859 nm and 551 nm are observed respectively when pumping fs pulse with central wavelength at 850 nm. The experimental result agrees well with the theoretical prediction on the anti-Stokes signal wavelength with only 3 nm divergence. The ratio of anti-Stokes signal power to the residual pump power is as high as 73.