AbstractExcess water pressure strongly affects the rheology and flow behaviour of debris flows, and the mechanisms for development and persistence of the pressure can be diverse. Using nine experimental slurries prepared with the sediments collected from the deposit of a large‐magnitude debris flow by modifying maximum grain size and solid concentration, the influence of agitation on excess water pressure is investigated with a cylinder apparatus, consisting of a coaxially mounted bucket and impeller driven by a variable‐speed motor. Agitation proceeding in three dynamic stages raises relative excess water pressure (R) by 17%–34% of the peak R in the static stage, showing that agitation can significantly increase excess water pressure. The magnitude of the steady R increases with the level of agitation, which is measured by impeller speed and slurry temperature, and the steady R in a slurry is agitation‐level‐dependent below its potential maximum value. The transfer of particle weight to water in debris flows can occur under zero or very low to extremely low effective stresses. Agitation raises excess water pressure essentially by promoting dispersion of the finest‐grained grains in water and that of progressively larger grains in gradually coarser‐grained slurries. The flow resistance of the tested slurries, characterized by net torque exerted on the impeller by the motor, decreases with increasing liquefaction ratio, demonstrating that agitation enhances debris‐flow mobility by raising excess water pressure. It is of importance to dampen agitation by building proper mitigation measures in remedial practices.