Optical-resolution photoacoustic microscopy (OR-PAM) can image the blood oxygen saturation (sO2) in vivo without labeling. OR-PAM assumes a linear relationship between the photoacoustic amplitude and the optical absorption coefficient and ignores the wavelength-dependent optical fluence attenuation in tissue. However, strong scattering in biological tissues may significantly change the optical energy deposition, leading to inaccurate sO2 measurement. Here, we report fluence-compensated OR-PAM to correct the sO2 imaging. In a narrow optical spectrum, we assume the scattered fluence is linearly related to the optical wavelength. Using three optical wavelengths, we can compensate for the scattering-induced photoacoustic signal change and thus improve the accuracy of sO2 measurement. We use a Monta Carlo model to validate the linear assumption of the scattered fluence. In in vivo experiments, we demonstrate that the optical fluence compensation can effectively improve the sO2 accuracy. The compensated arterial sO2 values are in the range of 0.95 ∼ 0.99, which is consistent with normal physiological values. Compared with the uncompensated ones, the accuracy has been improved greatly. Enabled by the accurate sO2 imaging tool, we can reliably observe the sO2 gradient in the vascular network. We expect this new technique will further broaden the preclinical and clinical applications of functional OR-PAM.