In the aim of revealing the creep properties of silty mudstone under complicated geological conditions, creep test was carried out on water-saturated silty mudstone of two different particle sizes collected from a mine under varying temperature and pressure condition by using multiphase coupling creep test device and acoustic emission monitoring system. Results suggest that the internal structure and the external geological environment both exert remarkable influence on the long-term mechanical behavior of silty mudstone. With the decline of confining pressure, rising of temperature, and increase of grain size, the strength of silty mudstone reduced while steady creep rate increased. Steady creep rate decreased linearly with the increase of confining pressure, but augmented exponentially with the rising of temperature. Porosity changed in a similar trend with creep rate, both displaying a U-shape distribution. Under the same confining pressure and temperature, the larger the grain size of specimens, the greater the porosity changed. Moreover, with the rising of temperature and confining pressure, the acoustic emission signal displayed higher frequency and larger amplitude; signals in specimens of larger grain size were more evident.