The invention discloses a water-rich weak surrounding rock long-and-big tunnel construction method. The method includes that a tunnel inclined shaft is arranged on one side of a main tunnel of a tunnel to be constructed, the main tunnel is divided into a tunnel entry section, a tunnel exit section and a middle connecting section connected between the tunnel entry section and the tunnel exit section, the middle connecting section is divided into an inclined shaft assisting construction entry section and an inclined shaft assisting construction exit section, and each of the tunnel entry section, the tunnel exit section, the inclined shaft assisting construction entry section and the inclined shaft assisting construction exit section is divided into multiple sections for construction. A construction method of each section includes: step 1, performing geological forecasting in advance, and when a debris flow formation exists ahead, entering a step 2, or else, entering a step 3; step 2, performing stability control construction; step 3, determining a primary supporting scheme; step 4, excavating the tunnel, and performing primary supporting construction; step 5, performing secondary lining construction on the tunnel. The water-rich weak surrounding rock long-big tunnel construction method is simple in step, reasonable in design, simple and convenient to construct, good in using effect, capable of meeting construction requirements of water-rich weak surrounding rock long-and-big tunnels, short in construction period and safe in construction process.