A new method named Digital Phosphor technology (DPX) is presented for real-time spectrum analysis and signal detection. The process of generating the 3-D spectrum in the DPX system is analyzed, and a practical DPX system is designed and implemented. The maximum effective bandwidth is 500MHz, and the signals are sampled directly in radio frequency (RF) with 1.2GHz sampling rate. The main data processing module employs dual-FPGA architecture. 65,535 frames of frequency waveforms are continuously processed and accumulated in one DPX image within 100ms. The experimental results have indicated that the 3-D spectrum of DPX greatly improves the capabilities of signal discovery, capture and analysis, and the DPX system is adapted to real-time spectrum analysis and signal detection.