Fiber lasers are impacted by temperature in various ways. This article studies on thermal characteristic of absorption cross section, luminescence cross section and lasing wavelength and describes the thermal impact on fiber lasers in details. As the temperature varies, the distribution of the particles in all sub energy levels changes, which makes the absorption cross section and luminescence cross section different. The relationship between temperature and absorption cross section is calculated by a fitting method. The luminescence cross section is the function of temperature and absorption cross section by which the luminescence cross section in different temperatures can be calculated. Relationships between lasing wavelength and temperature, fiber length, cavity loss, doping concentration are deducted. Furthermore, the article compares the numerical result of lasing wavelength under different temperatures with the experiment data, which match with each other well.