A time-resolved electron paramagnetic resonance (TREPR) method with 40 ns time resolution and a high sensitivity suitable for the detection of short-lived radicals under thermal equilibrium is developed. The key is the introduction of a new detection technique named ultrawide single sideband phase sensitive detection (UWSSB-PSD) to the conventional continuous wave EPR, which remarkably enhanced the sensitivity for the detection of broadband transient signals compared with the direct detection protocol. By repeatedly triggering a transient kinetic event f(t) (e.g., by laser flash photolysis) under a 100 kHz magnetic field modulation with precise phase control, this technique can build an ultrawide single sideband modulated signal. After single sideband demodulation, the flicker noise-suppressed signal f(t) with an ultrawide bandwidth is recovered. An UWSSB-PSD TREPR spectrometer prototype has been built in our laboratory, which integrated timing sequence control, laser flash excitation, and data acquisition systems as well as the UWSSB-PSD algorithm with a conventional CW-EPR. It exhibited excellent performance in monitoring a model transient radical system, laser flash photolysis of benzophenone in isopropanol. Both the intense chemically induced dynamic electron polarization (CIDEP) signals and the much weaker thermal equilibrium EPR signals of the generated acetone ketyl radical and benzophenone ketyl radical were clearly observed within a wide timescale (ranging from sub-microsecond to milliseconds). This prototype validated the feasibility of the UWSSB-PSD technique and demonstrated its superior performance in studying complex photochemical systems containing various transient radicals, which complements the established TREPR techniques and provides a powerful tool for deep mechanistic understandings, such as in photoredox catalysis and artificial photosynthesis.