Abstract: Estimation of output power for high output power single frequency He-Ne laser of self-stabilized frequency is introduced.The theory estimation results of output power much approximate to measurement data.Two series of single and self-stabilization frequency high output power external cavity He-Ne laser are obtained with inhomogeneous longitudinal magnetic fields and high gas pressure.With the discharge tube of one meter (cavity length 1140mm) and 1.8 meters length (cavity length 1800mm) the output power are 18～24mW and 40～52mW respectively.The coherence length of these lasers exceeds 14 meters.Whithout any external frequency stabilization method the instability of the single frequency are 6.175×10-9 (τ≤10s),3.38×10-9 (τ≤1s) for one meter laser,and 2×10-8 (τ≤10s),1.1×10-8 (τ≤1s) for 1.8 meters by multi measurements of the beat frequency between the single He-Ne laser and an iodine stabilized He-Ne laser.