Plant potentials are ultra-weak signals, sometimes its amplitude is only about 10～20 μV, and the weak parts of potential are easily submerged by noising. This paper introduced the construction and technology in developing the measuring system for plant electrical potentials, the input impedance of the amplifier must be high enough (≥10¹⁰～10¹²Ω) in order to measure the plant weak signals and ensuring high common mode rejection ratio (CMRR). In signal processing, wavelet transform was applied in decomposing and reconstructing based on multi-resolution wavelet analysis and in denoising based on cross-validation (CV) threshold value. Furthermore, the paper gives an application example in measuring plant potentials, the experiment shows that this system could effectively detect the change of plant potentials. Four-week old cucumber seedlings were stimulated by heat injury, the variation potential (VP) and action potential (AP) would be evoked, the AP varying amplitude can reach 40 mV; the AP changed very fast, its duration time is about 0.01～0.02 s and its amplitude often changes within 15 μV～30 mV.