The invention discloses a tool for detection of soil water content and especially relates to a fast detector of soil water content and a detection method therefor. The fast detector comprises a stainless steel housing (1). A receiving chamber (2), magnetrons (3), an infrared lamp (4), an automatic weighing device (5) and a weighing support (6) are installed in the stainless steel housing (1). The weighing support (6) is installed in the receiving chamber (2), passes through the top of the receiving chamber (2) and is connected to the stainless steel housing (1) by a connection rod. The automatic weighing device (5) is arranged on the connection rod. The magnetrons (3) are installed at outer side walls of the receiving chamber (2). The fast detector avoids the influence produced by factors such as natural water loss in transport on detection accuracy, avoids the influence on the whole engineering progress, effectively prevents soil sample burst in heating, guarantees soil sample integrity and improves detection accuracy.