Abstract: Agriculture and forestry conversion refers to land conversion between agriculture and forestry. This phenomenon is widely seen especially in the tropics. It is comprised of two main parts, namely, agricultural advance and forest retreat (AAFR), and agricultural fallow and forest recovery (AFFR). Agriculture and forestry conversion has dominated land cover and land use change in Mainland Southeast Asia (MSEA) in the past decades. Fire, including wildfire and man-made fire, plays a critical role in converting primary and secondary forest into cropland. By contrast, the practice of slash and burn also affects the recovery of forests in the tropics. Against the backdrop of climate change (e.g., high temperatures or drought), the risk of fire, especially extreme fire, is markedly elevated. Under the interplays of growing population, rapid economic development, and tighter land use policies, coordinated management of agro-forestry conversion and active fires is becoming increasingly important. Meanwhile, the process coupling and multi-scale effects between active fire occurrence and land cover change caused by agro-forestry conversion are important issues in climate change. A number of publications have focused on exploring the contributing roles of the scale, intensity, and process of cropland and forest conversion at the regional level. However, the contribution of active fires to land conversion between agriculture and forestry in the entire MSEA remains poorly investigated. With the datasets of Moderate Resolution Imaging Spectroradiometer (MODIS) land cover and fire products (active fire and burned area) during 2002-2021, the study reveals the coupled association between active fires and cropland-forest conversions in MSEA, and then evaluates the contributing role of active fire occurrence and development in agricultural advance and forest retreat using the GIS fishing nets, kernel densities, and Pearson correlation analysis. The results show that: (1) Under the impact of active fires, agricultural advance and forest retreat are widely seen in MSEA, particularly in the national border areas and major riverside areas. The degrees of land cover change in the southern lowland plains are significantly greater than those in the northern hilly and mountainous regions. (2) The intensity of active fires, quantified through the statistics of fire occurrence frequency (FOF), fire radiative power (FRP), and burned area (BA), is positively correlated with the degree of agricultural advance and forest retreat, and negatively correlated with agricultural fallow and forest recovery. Occurrence intensity of active fires shows positive correlation with the change scale of agricultural advance and forest retreat, and negative correlation with the change scale of agricultural fallow and forest recovery. (3) Fire is an important cause of land cover change between cropland and forest in MSEA, which significantly affects its direction and intensity. MODIS 1-km active fires account for nearly 40% of the average multi-year contribution to deforestation and nearly 60% of the average multi-year contribution to cultivated land expansion. This study not only provides new evidence and understanding to explore the process and mechanism of active fire development from the land cover/use change perspective, it also provides guidance for sustainable forest utilization, agricultural production practices, and active fire management in Mainland Southeast Asian countries.