Because conversion efficiency (CE) can characterize ultraviolet (UV) down-conversion light-emitting thin films accurately which is very useful in photoelectric devices, the research of CE is very crucial. An experimental determination of conversion efficiency of UV down- conversion light- emitting thin films based on using an integrating sphere to collect the photons is proposed, which solves the problem owing to disregarding anisotropy and wave-guiding effects in determining the angular distribution of the emission. A new computing method by using the emission spectrum of samples is also proposed, which is easy to compute. Demonstration experiments of Lumogen and Coronene, two fluorescent materials often applied in UV down- conversion field are made. Demonstration experiments show that this method is more reliable and more accurate, which should be recommended for computing CE in UV down-conversion research field.