Immunoassays contribute greatly to food safety. But there are no reported immunoassays that simultaneously detects MQCA and QCA, the marker residues for olaquindox and carbadox, respectively. Here a broad-specificity MAb was successfully produced, and the MAb showed good cross-reactivity with both MQCA and QCA, having IC50 values in buffer of 4.8 and 9.6 ng/mL, respectively. The calibration curves ranged from 0.3 to 81 μg/kg. The average recoveries ranged from 76 to 108% at different spiked levels (2, 4, and 8 µg/kg for MQCA; and 4, 10, and 20 µg/kg for QCA), and the intra/inter-day coefficient of variations were 4.2-13.3%. The limits of detection of MQCA and QCA in chicken, fish, pork, and shrimp were 1.76, 1.32, 1.90, and 1.18 µg/kg, respectively. This method was verified by LC-MS/MS, with a correlation coefficient above 0.98. The immunoassay was rapid and reliable for simultaneous screening analysis of MQCA and QCA residues.