A spread equation of erbium-doped optical fiber laser was derived from the basal theory of laser. The experimental results showed that erbium-doped optical fiber exhibited gas gain when the wavelength was 1550 nm, corresponding to a low-loss communication window, possessed potential application value. It was very important parameter for the laser that influencing factor of overlap integral of the distribution of pumping fiber mode intensity and erbium doped fiber ions. Whether mode waist overlapped with the distribution of erbium-doped fiber ions was determined by various mode fields. This might contribute to an analyse of fiber mode. Influencing factors of overlap integral with medium absorption cross section were investigated. Theoretical analysis of the noise characteristics (ASE) was conducted. the analytical formula of gas gain, threshold power and slope efficiency was prescribed and a numerical model with output characteristic was developed. It were obtained by working out special computation procedure for this laser, performing theoretical arithmetic and emulation for output characteristic and discussing the effect of the configuration.