The utilization of powdery semi-coke as a power fuel in pulverized coal-fired power plants has become a new and potential technique to consume the excess powdery semi-coke. The characteristic of low volatile results in poor combustion performance and high NO x emission, and to co-fire with bituminous coal is a practical strategy to address this problem. However, the co-combustion characteristics and the inherent interaction between semi-coke and coal remain insufficiently understood. In addition, the influences of secondary air arrangement, the boiler operation load, and the fuel type on co-combustion process are still unclear, which is urgent to be further explored. In the present study, experiments and numerical simulations were jointly utilized to inquire into the co-combustion behaviors and NO x emission features of semi-coke and coal. The results demonstrated that the “out-furnace method” was a suitable choice for small-capacity boiler when the proportion of semi-coke was 33%, due to the limited combinations of the semi-coke injection position. It was recommended that semi-coke was preferred to be injected from the middle layers of the furnace under the “in-furnace method” to improve the overall co-combustion performance. The critical value of the separated over fire air ratio in this study was 27.5%, over which a slight drop of carbon content in fly ash could come about. Moreover, the elevation in the proportion of separated over fire air gave rise to the significant decline of NO x concentration. The constricted secondary air arrangement was preferred to be employed due to the high boiler efficiency. The separated over fire air and the surrounding air needed to maintain a wide-open degree to prevent the increase of NO x emissions and the coking of nozzles. For the load reduction regulation method adopted in this study, the NO x concentration first rose and then dropped, while the burnout ratio decreased obviously as the operation load was reduced. Different combinations of coal and semi-coke generated significant influences on co-combustion behaviors within the furnace. The NO x generated by high-volatile fuel (bituminous coal) combustion was mainly affected by volatile-N, while the NO x generated by low-volatile fuel (semi-coke) was mainly impacted by char-N. This study is of guiding significance for the efficient and clean utilization and beneficial to the large-scale application of powder semi-coke in power plants.