The invention provides a rectangular drilling stress monitoring device and method. The monitoring device comprises a rectangular drilling bit, a drilling pipe and a rectangular drilling stress monitor. The rectangular drilling bit includes an outer frame, and the outer frame is internally provided with forward cutters, diagonal cutters and a center guide cutter. The rectangular drilling stress monitor includes an upper cover, a lower cover, stress sensors and digital displayers. The monitoring method includes the steps of forming guide drilling holes in advance, forming rectangular drilling holes and installing the rectangular drilling stress monitor for monitoring. The monitoring device can effectively overcome the shortcomings that the test direction of traditional drilling stress gaugesis unclear and the test value is inaccurate, can accurately measure surrounding rock stress values at any angle in the vertical direction and the horizontal direction and can provide more reliable data for on-site mine pressure disaster prediction and forecast. The device and the implementation method provide a more reliable guarantee for monitoring of surrounding rock stress in coal mines in China.