We report a propagation spectrum detection system in which one end of a plasmonic silver nanowire is locally illuminated from a normal halogen lamp and the scattered light is recorded spectroscopically at the other end. The system is applied to investigate surface plasmon polariton-Fabry-Perot (SPP-FP) modes of silver nanowires with different lengths at air-glass and oil-glass interfaces. The generalized FP model is used to analyze the spectrum, which fits well with the experimental results. The influence of nanowire length and environment on the properties of the FP resonances is discussed. The propagation spectrum detection system will find applications for integrated optical circuits and plasmonic sensing.