We propose an active method for light field depth computation with high accuracy and high robustness. Structured illumination is employed to provide encoding information so that both direction information of light rays and depth-modulated phase information are recorded in the light field. The phase information can provide intensity-insensitive matching features for constructing light field depth cues. Thus, scene depth can be computed accurately. Experimental results demonstrate that, compared with the passive method, the proposed active method can obtain higher quality depth maps and achieve light field depth computation with higher accuracy and higher robustness.