Due to the structural characteristics of the constrained space and the poor heat resistance and abuse resistance of lithium-ion batteries (LIBs), the thermal runaway (TR) risk of LIBs is greatly increased in the confined space. In this work, experimental methods are mainly employed to study the effect of spacing on TR and smoke temperature of double 32,650 lithium iron phosphate (LFP) batteries. The combustion process of double LFP batteries is divided into nine phases based on observed phenomenon. When the second jet fire occurs in A, the TR of double LFP batteries reaches the most intense state. The highest flame temperature is up to 700 °C, and the maximum smoke temperature of ceiling caused by jet fire can reach 110 °C. When the battery spacing is small, the phenomenon of flame fusion occurs in the stable combustion phase. The flame height is significantly improved of about 30%, while the temperature at the center of flame fusion does not increase. As the battery spacing increasing, the thermal radiation and thermal convection between double LFP batteries weaken, and the flame height shows a downward trend. The duration of the stable combustion phase is increased, and the temperature at the positive electrode tabs shows an overall upward trend. When the battery spacing exceeds 1.5D, it can be assumed that battery spacing has little effect on the interaction between double LFP batteries. The results of this work provide a research basis for assessing the TR risk of LIBs under a narrow and long constrained space.