A test system is designed for evaluating the characteristics of single-spectrum radiant energy-electricity energy conversion of thermophotovoltaic cells. A band-pass filter and a secondary concentrator with a straight funnel are used to achieve single-spectrum and directional radiation transmission between the xenon lamp and the thermophotovoltaic cells with adjustable energy flux density. Based on Monte-Carlo ray tracing method, the spectral radiation transmission model of the xenon lamp-thermophotovoltaic test system is constructed. The distribution characteristics of radiant energy flux on the cell surface are analyzed, and the effects of opening size and height of the secondary concentrator are discussed. Meanwhile, the current-voltage curve, output power, and thermoelectric conversion efficiency of GaSb cell under single-spectrum and high energy flux radiation are theoretically calculated. The results show that the introduction of a secondary concentrator can effectively improve the thermoelectric conversion performance of the thermophotovoltaic cells. The radiant energy on the cell surface increases by 105.3%, with the uniformity reaching 95%, and the output power of the cell increases by 109.1%.