The experimental results of the second harmonic emission (SHE) in an ultrashort (45fs, 6×10 17 W/cm 2) laser pulse interaction with underdense plasmas formed from an optically ionized helium gas are presented. The spectra of SHE are measured under different intensities of fs laser pulses and the gas densities. The relationship between the conversion efficiency of SHE and the laser energy is studied. The theoretical explanation is given to the origin of SHE in underdense plasma. In consideration of the radial electron density gradient in plasmas produced by the intense ultrashort pulses propagation in the gas, a good agreement is obtained between experimental results and theoretical curve based on the nonlinear process.