Three-dimensional (3D) photoacoustic imaging can be achieved by a two-dimensional (2D) ultrasound array matrix. However, the 2D matrix consisting of hundreds or thousands of transducer elements makes it not only expensive, but also a big technical challenge for both probe manufacturing and parallel data acquisition. In this study, we performed the photoacoustic imaging by scanning an unfocused linear ultrasound array probe over a planar geometry, resulting in an equivalent 2D matrix probe. The phantom study demonstrated that this method substantially increased imaging quality, which has great potential for animal and clinical photoacoustic imaging.