The accurate and sensitive monitoring and imaging of mitochondrial pH in living cells play vital roles in chemical biology and biomedicine. Herein, we design a novel ratiometric fluorescent chemical probe for monitoring and imaging the pH of mitochondria in living cells based on congo-red (CR)-modified dual-emission semiconducting polymer dots (Pdots) via a competitive fluorescence resonance energy transfer (FRET) mechanism. The Pdots are synthesized by triphenylphosphonium (TPP)-modified polyoxyethylene nonylphenylether (CO-520), poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO), poly(9,9-dioctylfluorene)-co-(4,7-di-2-thienyl-2,1,3-benzothiadiazole) (PF-DBT5), and poly(styrene-co-maleic anhydride) (PSMA) via a nanoprecipitation method, and the prepared Pdots are further chemically linked with pH-sensitive, nonfluorescent CR to obtain the mitochondria-targeted pH fluorescent probes. This Pdots-based probe consists of two luminescent components including PFO and PF-DBT5 as fluorescence donors, an acid-base indicator CR as an energy acceptor, and TPP as the mitochondria-targeting group. At a different pH region, the FRET efficiency between CR and PFO or CR and PF-DBT5 can be modulated. This probe exhibits good biocompatibility, a wide pH detection range from 2.57 to 8.96, good reversibility, high selectivity, and excellent photostability for pH monitoring. This probe allows for the detecting and imaging of mitochondrial pH in living cells with satisfactory results.