In this research, we attempted to develop a sensitive colorimetric sensing strategy for the detection of acid phosphatase (ACP) based on MnO nanosheets and explored its applications in screening and evaluating inhibitors of ACP. The MnO nanosheets exhibit intrinsic biomimetic oxidase activity, which can catalyze the oxidation of the colorless 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonate) diammonium salt (ABTS) into green oxidized ABTS (oxABTS). Upon the introduction of ACP, l-ascorbic acid-2-phosphate can be dephosphorylated to ascorbic acid, which arouses the disintegration of MnO nanosheets into Mn ions. This disintegration weakens the enzyme mimicking activity of the MnO nanosheets, leading to the impediment of the oxidation of ABTS. Conversely, in the absence of ACP, the ABTS is rapidly oxidized by MnO, leading to a significant colorimetric signal change. The absorbance difference at 420 nm displayed a linear relationship with the concentration of ACP ranging from 0.075 to 0.45 mU·mL, generating a detection limit of 0.046 mU·mL. In the inhibition assays, this sensing platform provided simple detection for parathion-methyl (PM), a representative inhibitor of ACP. The facile evaluation of the inhibitory effect of PM, including its IC50 toward ACP, was also realized.