On the basis of the rate equation and the optical transmission equation for a double clad fiber laser, a mathematical model was established to calculate the output power for the optical fiber laser. Then, the distribution of output power along optical fiber and distributions of output power and pump power in different fiber lengths were simulated numerically for a neodymium doped fiber laser. By taking a 808 nm semiconductor laser as pumping source. By taking a neodymium doped double clad fiber as gain medium and KTP crystal as a multiplier, the efficiency of doubling frequency and phase angle of the laser were calculated and simulated. Finally, the frequency doubling of fiber lasers was simulated. Experimental results show that the fiber laser can achieve visible light output in high efficiency.