The varying principle of workability of ultra-high performance concrete (UHPC) slurry can be theoretically studied with the relationship of its workability and rheological properties. An orthogonal test was designed to study the effects of water-binder ratio (W/B), substituted rate of cement by superfine fly ash and silica fume content on the workability and rheological properties of UHPC slurry. The film thickness on particles was taken as the index to evaluate workability and rheological properties of UHPC slurry. According to the test results of workability and rheological properties, the combined effect of W/B and substituted rate of cement by superfine fly ash on the workability and rheological properties of UHPC slurry were analyzed, and the relationship of workability between paste and mortar was investigated. Based on the film thickness on particles, the relationship between the workability and rheological properties of UHPC mortar was established. The results show that W/B is the most important factor, and as W/B, substituted rate of cement by superfine fly ash and silica fume content substituted increase, the film thickness on particles increases. Flow spread and consistency coefficient of UHPC slurry are correlated due to the combined effect of W/B and substituted rate of cement by superfine fly ash.