Abstract: A two-dimensional continuous surface sub-wavelength grating was designed on the organic light-emitting diode(OLED) substrate surface in order to improve the efficiency of OLED. The sub-wavelength grating with approximate continuous surface structure was design by the method of equal height step approximation, according to the theory of equivalent medium and the principle of thin film optics. The grating parameters were designed as follows: the etching depth is 0.29 μm, the period is 0.165 μm, the ratio of the bottom diameter to the period is equal to 1, which can effectively realize high transmittance of wide spectrum and wide angle. The effect of the grating on the light efficiency of the OLED substrate was simulated by the finite-difference time domain method, and the results show that the maximum light output efficiency of 30% can be increased.