Mn and Si are common elements in steel and have an important influence on the performance of steel. To analyze the intermetallic compounds formed in tin-base solder welding process, the Mn-Si-Sn alloys were prepared and annealed at 500 degrees C for 30 days. The phase equilibria in the Mn-Si-Sn system at 500 degrees C were studied by electron microscopy equipped with energy-dispersive spectrometry and x-ray diffraction. 12 three-phase regions were well determined in the Mn-Si-Sn system at 500 degrees C, and no ternary compound was found. The binary compound Mn5Si2, which is controversial in the literature, was not observed in the present study. Mn2-xSn (Mn1.8Sn) and Mn3Sn2 phases were detected by XRD rather than the Mn2Sn phase in this work. The solubility of Si in Mn3Sn, Mn2-xSn, Mn3Sn2, and MnSn2 was measured to be 0.7, 1.2, 1.2, and 0.4 at.%, respectively. And the Sn solubility in R-Mn6Si, upsilon-Mn9Si2, Mn3Si, MnSi, Mn11Si19 at 500 degrees C is about 0.9, 0.5, 0.3, 0.3 and 0.2 at.%, respectively.