The precursor brightenings of solar flares hold valuable clues concerning the flare triggering and energy release mechanisms, but have not yet been extensively studied. This paper presents a multi-instrument study of the two precursor brightenings prior to the M6.5 flare (SOL2015-06-22T18:23) in NOAA active region (AR) 12371, with a focus on the temperature (T), electron number density (n) and emission measure (EM) of these two precursors. The data used in this study were obtained from four instruments with a variety of wavelengths, i.e., the Solar Dynamics Observatory’s (SDO) Atmospheric Imaging Assembly (AIA) in six extreme ultraviolet (EUV) passbands, the Expanded Owens Valley Solar Array (EOVSA) in microwave, RHESSI in hard X-ray (HXR) and GOES in soft X-ray (SXR). We compare the temporal variation of T, n and EM derived from different data sets during the precursor period. Here are the key results: (1) EMs are more uniformly distributed over the passband from SXR to EUV. (2) RHESSI HXR and EOVSA microwave have similar high temperature response feature due to different causes. (3) AIA detects emissions primarily in high temperature range of EUV passband. We discuss their relationship and differences in terms of the sensitivity of the instruments and underlying physics.