Based on the analyzing the principle of fiber loop mirror,the resonant conditions of phase and intensity of Linear Cavity Er³⁺ doped Fiber Laser Based on Fiber Loop Mirror are obtained. The output characteristics of continuous Er³⁺ doped linear cavity fiber laser based on fiber loop mirror have been theoretically studied and numerically analyzed by solving rate equations,and analytical expressions on laser output power,threshold pump power and slope efficiency under stationary condition have been derived. The expression of minimum and optimum fiber length for designing a linear cavity Er³⁺ doped fiber laser has been given,which will be helpful to the optimization of designing a practical linear cavity Er³⁺ doped fiber laser of the same kind based on fiber loop mirror. 1550 nm maximums fiber laser output of slope efficiency 18.3% have been obtained in the experiment.