Abstract: The study of rheological mechanical properties of weak rock mass is of great significance for ensuring the surrounding rock stability of soft rock roadway in mine. It is necessary to carry out the field rheological test on weak rock mass and the research on its constitutive model. The in-situ graded-loading creep test on large-volume weak rock mass samples in the soft rock roadway in a coal mine was carried out. The fractional derivative theory was introduced. The Abel viscous element was used to replace the viscoelastic body (Kelvin body) in the traditional Burgers rheologi- cal model. Based on the deformation characteristics of rock mass samples during the accelerated creep stage in the test results,the nonlinear visco-plastic body (NAVPB) was connected in series to the improved Burgers model to establish a nonlinear fractional derivative creep model of the field weak rock mass. Based on the creep test data of the rock mass samples,the parameter fitting and comparative analysis of the nonlinear fractional derivative creep model and the Bur- ger model were performed. The field rheological experimental results of weak rock mass show that whether the rock mass samples undergo accelerated creep is related to the stress level of loading. There is a stress threshold of acceler- ated creep. When the stress is no more than the threshold,the rock mass sample only undergoes the decay creep stage and the stable creep stage,and the deformation rate in the stable creep stage is not zero. When the stress is greater than the threshold,the rock mass sample undergoes the complete decay creep,stable creep and accelerated creep three stages and fails finally. There is a startup point for the accelerated creep. Model parameter fitting and comparison re- sults show that the structure of proposed nonlinear fractional creep model of weak rock mass is simpler. Compared with the Burgers model,the proposed model can describe the full-stage creep characteristics including the accelerated creep stage of field weak rock mass. The research results have a certain application value to related projects.