Recently,high power linearly polarized narrow linewidth fiber lasers are widely applied in coherent detection and wavelength beam combining.Inhigh power linearly polarized narrow linewidth fiber lasers,the effect of mode instability(TMI) is one of the mainfactors limiting its power increase.In this paper,the influence of TMI effect on the output power of high power linearlypolarized narrow linewidth fiber laser is analyzed,and the suppression method of TMI effect is proposed.Long wavepumping technologies are used in this article.A single frequency laser with an output power of 100 mW is used as theseed source.And the phase modulator broadens the linewidth of the seed source to 23GHz.After three stageamplification,the linewidth of 23GHz,power of 2.2 kW,and center wavelength of 1064nm are finally realized.Linearlypolarized narrow linewidth fiber laser output with extinction ratio of 98% is achieved.Beam quality is M2x=1.2 and M2y=1.21.The influence of the pump wavelength on the TMI effect is analyzed.Due to the small core diameter of thefiber(20μm),a high absorption coefficient of the gain fiber for the pump light(1.8 dB/m@976 nm),the core temperatureis high.And the heat introduced by the pump photo quantum defect,causes the refractive index of the fiber core tochange.Finally,the TMI effect occurs at lower power.When the pump wavelength is shifted to the long wavelength,thequantum defect of the pump light and the pump absorption coefficient are both reduced.The heat distribution on theentire length of the fiber or on the unit length is reduced.The TMI threshold is increased.And the output power of thelinearly polarized narrow linewidth fiber laser is improved.