The relatively low sensitivity is the limitation of ELISA in early screening of diseases. Various signal enhancing methods are developed to increase detection sensitivity, but most of them require professional operation and are not compatible with commercial reagent kits or automatic analysis instruments, limiting their application in clinical testing. Here, we present a contactless and user-friendly ball-lens assisted fluorescence enhancing (BFE) method to improve immunoassay sensitivity. The BFE effect is observed in experiment and demonstrated by the simulation. Based on the BFE effect, the detection sensitivity of the immunoassay is improved about 3.6-fold and the limit of detection is lowered by more than 3-fold using commercial reagents and standard ELISA processes. In addition, the BFE effect causes no damage to the linear dependence, specificity and accuracy of clinical plasma measurement. The established method shows a high compatibility with automatic microfluidic immunoassay systems and other signal enhancing techniques, displaying great potential in multi-technique coupling assays and clinical applications.