The invention relates to the technical field of railway tunnel construction, specifically a digging method of IV and V-class surrounding rock railway tunnel with large cross section, aiming to solve the problems of the existing railway tunnel digging method including narrow operation space, low work efficiency, inflexible construction method, instable construction progress, high deformation potential of surrounding rock, and inconvenient operation. The method comprises performing leading support, digging arc-shaped pilot tunnel and preserving core soil; staggeredly digging left/right side walls of middle sidestep and preserving core soil; staggeredly digging left/right side walls of lower sidestep and preserving core soil; sequentially digging upper, middle and lower sidesteps and preserving core soil; and digging inverted arch and carrying out preliminary support. The invention has large construction space and high work efficiency, and can perform multi-job parallel operation. When the geologic conditions are changed, the invention can be flexibly and timely converted to other construction method. Preservation of core soil and left/right staggeredly digging are helpful to the stability of the digging working face. When the surrounding rock has large deformation, the invention can adjust closure time as soon as possible under the prerequisite of assuring safety and meeting requirements of clearance cross section.