Many methods are considered for calculating longperiod fiber gratings (LPFGs) transmission spectra such as integral method, formula method and transfer matrix method. In this paper, these methods are analyzed based on the coupledmode theory and their characteristics and the conditions of application are pointed out. Exact solution for multimode transmission spectra can be obtained directly by integral method in a wide wavelength range, however, the computation process of integral method is complicated. The formula method is convenient for calculating and expressing, but it can only solve single mode transmission spectra directly in a narrow wavelength range. The transfer matrix method is suitable for calculating some transmission spectra of nonuniform fiber gratings, it is simple to implement, almost always sufficiently accurate, and general the fastest. Through the analysis and compare of the three methods, a best method with high precision and simple calculation is provided for longperiod fiber gratings transmission spectra in theory, which is important for the research and application of LPFGs.