The light field camera can capture direction and intensity of light rays at the same time, when setting a lenslet array at a specific position between the primary mirror group and the sensor. We propose an image digital watermarking method for four-dimensional light field image obtained by the light field camera in the frequency domain. In this method, the embedded information is compiled by Base64 to generate a two-dimensional image, and then a virtual computational light field matching the original light field coordinates is established based on the image scrambled by Aronld cat map. A slice of frequency domain is obtained from virtual computational light field after the four-dimensional Fourier transform. By means of slice substitution, the information is embedded into original light field, in order to realize the aim of encrypting light field image. Finally, the watermark information is extracted successfully, by means of the Fourier inverse transformation and image binarization. A light field acquisition system is built, and a light field image is encrypted by the proposed method. The experimental results show that the encrypted light field has a high signal-to-noise ratio and strong correlation with the original light field. The proposed method is simple, fast, and reliable. There is no obvious artifact and distortion in the encrypted light field image.