Carbon monoxide (CO) is an important gas indicator for early detection and evaluation of spontaneous combustion in longwall gobs, and continuous monitoring of this gas is generally located at the upper corner of working face. Although CO emissions during spontaneous heating of coal have been investigated adequately in laboratory, few studies have involved in the migration and generation behavior of CO in longwall gobs. The CO-based early warning threshold for spontaneous combustion in gob is unsettled either. In this study, a fully coupled model of generation and transport of CO in longwall gob is developed by combining with air seepage, oxygen transport, thermal transfer, and exothermic reaction. Standard CO generation rate (SCOGR) as one key parameter is introduced for the better simulation on CO migration in gob. Our previously developed solving software of COMBASS-3D has been improved to investigate the functional relationship between progression of spontaneous combustion in longwall gob and CO emissions at upper corner. The results show that (i) the SCOGR depends strongly on temperature and coal properties, but is independent of ambient oxygen concentration, which is the basis for calculating actual CO generation rate; (ii) the high CO concentration zone overlaps with the oxidative self-heating zone and the high temperature zone in windward side of gob, which is consistent with the theoretical expectation and on-site observation; (iii) increasing longwall advance rate, decreasing ventilation flux and reducing thickness of abandoned coal all can not only reduce risks of spontaneous combustion in longwall gob but also suppress CO emissions from the gob; (iv) the maximum temperature in gob grows linearly with the increase of natural logarithm of CO concentration at upper corner in low temperature stage, and the early warning threshold can be calculated on basis of this relationship with the minimum self-heating temperatures of coal. These works can provide a quantitative approach for early warning the spontaneous combustion fires in longwall gobs.