Photoacoustic imaging is a hybrid imaging method that integrates optical contrast and ultrasound penetration, reserving the advantages of both the optical imaging and the ultrasonic imaging. In acoustic-resolution photoacoustic imaging (AR-PAM), focused transducers with large numerical are usually applied to AR-PAM to achieve high image quality. However, the lateral resolution and signal-to-noise ratio of photoacoustic images degrade significantly in the out-of-focus region due to the limited focal depth of transducer. To address this challenge, algorithms of synthetic aperture focusing technology (SAFT) based on virtual point concept have been developed. These include one-dimensional-SAFT, two-dimensional-SAFT, adaptive-SAFT, spatial impulse response-based scheme, and delay-multiplication-sum solution. This review introduces the advantages and limitations of these algorithms, and discusses their outlooks for future developments in AR-PAM.