Absorbers locating in the area of finite focal depth can be imaged with high resolution and signal-to-noise-ratio (SNR) using high resolution photoacoustic microscopy, whereas contours of actual biological tissues turn out to be irregular, causing non-uniform resolution and SNR in photoacoustic images. Sliding focus method based on sparse contour scanning is introduced, first scan contour sparsely based on fixed focus, manually choose contour locations, then obtain the full contour matrix using biharmonic interpolation, which is then transformed to sliding focus matrix, second scanning is done in the last, during which the focus location is adjusted before each scanning step to make sure the absorbers locate in the area of focal depth, thereby images with uniform resolution and SNR can be obtained. Phantom experiment and in vivo blood vessels imaging are conducted to prove the validity of the method.