The UV curing behavior of a highly branched polycarbosilane (HBPCS) was studied by differential scanning photo calorimeter (DPC) measurements. 2-Hydroxy-2-Methyl-Phenyl-Propane-1-one (Photocure-1173) was selected as photoinitiator. In order to accelerate the curing reaction rate of polycarbosilane, acrylic reactive diluent tripropane glycol diacrylate (TPGDA) was also added to the polymer system. The effect of TPGDA content, photoinitiator concentration, temperature, light intensity, and the curing atmosphere was investigated. The results indicated that the TPGDA greatly enhanced the reaction rate of the HBPCS system. The TPGDA content, photoinitiator concentration, temperature, and light intensity had their own optimal values to get the maximum ultimate conversion percentage and the reaction rate. The oxygen atmosphere helped to increase the final conversion percentage though it could retard the reaction rate.