Based on the aperture averaging effect of the scintillation effect of light propagation, a large aperture laser scintillometer (LALS) for measuring the refractive index structure constant of atmospheric turbulence was developed. This apparatus can restraint the scintillation saturation at certain extent. This scintillometer has symmetrical structure. Meanwhile, signal modulation and demodulation method is used to increase signal-to-noise ratio dramatically and the embedded control unit is introduced to sample, dispose, display and store the data. Compared with the measuring results from traditional measurement device, the slope of linear regressions of the data in 1000 m horizontal propagating path is 1.01 and the correlation coefficient is 0.92. Further analysis reveals that the inner scale of the atmospheric turbulence which will influence the measurement of the scintillation variance may lead some degree of scattering of the measurement results of the refractive index structure constant.