The conventional extension degree cannot fully reflect the construction performance of ultra-high performance concrete (UHPC), and it should be supplemented by the rheological properties. However, the proprietary equipment for the measurement of rheological properties is expensive, which is inconvenient to be directly used on the construction site. Therefore, 18 kinds of mix proportions of UHPC were prepared via changing the water-to-binder ratio and superplasticizer dosage. In this work, the flow and rheological parameters of various mix proportions of UHPC were obtained via testing the extension degree, characteristic time of extension degree (i.e., the time required for extension diameter of 0.7 and 0.8 times), rheological behavior, and mechanical properties after curing of the corresponding mix proportions. Finally, a relationship between fluidity and rheology of fresh UHPC (without steel fibers), which the mechanical properties belong to UHPC (with steel fibers), was established. The results show that the Bingham model can be used to describe the rheological behavior of fresh UHPC. The proposed characteristic time of extension degree index has a relatively high test accuracy. The rheological parameters of fresh UHPC can be obtained from the flowability-rheology relationship established. It is indicated that the calculated data by the proposed method agree well with the experimental results.