A novel pH-responsive fluorescence probe, (4-morpholinomythyl) boronic acid (4-MPBA), was synthesized and employed for the detection of organophosphorus pesticides (OPs) in fruit juices. 4-MPBA was sensitive to pH changes in the range of 3.0–5.8 with R² of 0.9914, which was based on the inhibitory reaction between OPs and acetylcholinesterase (AChE). The linear range spanned over the range of 0.80–32 μg·L⁻¹ with limits of detection of 0.16–0.24 μg·L⁻¹. This fluorescence assay was utilized for detecting trace-level OPs with satisfactory recoveries (95.73–104.50%) and RSDs of < 6.74%. The total residues of OPs were detected to be 5.50–20.84 μg·L⁻¹ in 12 unfortified juice samples. The experimental accuracy and precision offered by this probe were verified to be satisfactory using a certified reference fruit juice (QC-202005A-1). Thus, we posit that the newly developed fluorescence probe can be satisfactorily applied for daily monitoring of trace OPs in juice matrices.