The numerical aperture is an important parameter of optical fiber, and the fiber with high numerical aperture can be well used in fiber laser and laser-induced fluorescence system. The numerical aperture of the photonic crystal fiber is different from that of traditional step optical fiber, which is closely related to the wavelength. In the present paper, a spectrometer was used to measure the numerical aperture of photonic crystal fiber, and a lot of refractive index photonic crystal fibers were measured and simulated to investigate the impact of wavelength, the diameter of air-hole and the pitch on numerical aperture. According to the measured numerical aperture, the parameters of fiber related with wavelength can be better studied, including nonlinearity coefficient, macro-bending loss, effective mode area, cut-off wavelength and so on, and satisfactory results were achieved.