A novel and selective analytical method has been developed for doxycycline (DOX) determination, addressing the critical need for monitoring this widely used antibiotic in environmental samples. The method employs a hybrid sensing system combining orange-emitting carbon dots (O-CDs) with a calcium-murexide (Ca@Mu) complex, offering a unique dual-mode detection approach. Unlike conventional methods that rely on direct fluorescence quenching by DOX, which often suffer from poor selectivity, this system utilizes the competitive binding between DOX and murexide for Ca²⁺ ions. Initially, the pink-colored Ca@Mu complex formed under alkaline conditions causes fluorescence quenching of O-CDs at 552 nm. Upon introduction of DOX, its stronger Ca²⁺-binding affinity leads to the formation of a calcium-DOX complex, liberating free Mu and triggering both colorimetric (pink to orange) and fluorometric (restoration of O-CDs emission) responses. Comprehensive characterization and mechanistic investigations employing different spectroscopic techniques confirmed the sensing mechanism. The method demonstrates excellent selectivity for DOX among other tetracyclines and potential interferents, along with impressive analytical performance including good linearity (1.0-35.0 μM), low detection limit (325 nM), and high precision. Additionally, a smartphone-based colorimetric platform was developed for convenient on-site analysis. The practical utility of this method was validated through successful determination of DOX in environmental water samples with excellent recovery rates, offering a reliable and user-friendly approach for environmental monitoring of this important antibiotic.