The effect of transverse mode instability has gradually become the primary problem that causes laser beam quality degradation and limits power scaling of high-power fiber lasers. This paper conducts a series of study on the transverse mode instability (TMI) in large mode area (LMA) fiber based on a co-pumped all-fiberized narrow linewidth high power amplification platform. According to the calculation results of the coupled mode equations, the nonlinear coupling strength between the LP₀₁ and LP₁₁ modes of the LMA fiber 25/400 μm is the largest, which directly induces the TMI. In order to suppress the generation and amplification of the LP₁₁ mode at the main amplifier, the fiber coiling method was used as an operational mode filtering technique to achieve mode control. The threshold of TMI increased from 1000 W to 1600 W while reducing the bending radius of the main amplifier gain fiber from 6 cm to 5 cm, and the other output performance of the fiber laser was hardly affected. This provides a powerful experimental reference for us to build an actual narrow linewidth, high power, all-fiberized laser system.