Abstract: One of the key points for developing ultra-high performance concrete(UHPC) with good construction is to determine the suitable water-to-binder ratio.Therefore, based on the premise that the typical raw materials and mix ratio of 150MPa grade UHPC remained unchanged(except water consumption), the influence of water-to-binder ratio(0.15~0.2) on the construction and mechanical properties of UHPC were studied. The extensibility of UHPC in different wet stirring time, full wet stirring time, extensibility of UHPC for a standing time after full wet mixing, compressive strength, flexural strength, four-point flexural stress-deflection curve, and the equivalent tensile stress at the lower edge of the test piece when the visible first crack occured, were obtained through extensibility, compressive strength and four-point flexural test. Subsequently, the variability of compressive strength and flexural strength results were analyzed; The flexural toughness index was calculated based on the four-point flexural stress-deflection curve and the standard method. At the same time, an improved method for calculating flexural toughness index of UHPC was proposed. The test results show that the full wet stirring time of UHPC is 6 min. The average extensibility rate of UHPC increases to 76mm with the water-to-binder ratio increases by 0.01. The extensibility loss of UHPC is 40mm, and the loss rate is only 5.9%, when the water-to-binder ratio and standing time are 0.19 and 4h. The compressive strength, flexural strength and flexural toughness index of UHPC all increase first and then decrease with increasing of water-to-binder ratio, and when the water-to-binder ratio is 0.18, 0.16 and 0.16 respectively, the three mechanical properties reach the best and the coefficient of variation is smaller. The highest average value of compressive strength and flexural strength of UHPC is 172.58MP and 42.07MPa under the same water-to-binder ratio, and the corresponding extensibility are 580mm and 360mm respectively. The optimal water-to-binder ratio for construction and mechanical properties of UHPC is 0.185(extensibility is 620mm, compressive standard strength is 158.54MPa, and flexural strength is 29.3).The stress-deflection curve before the stress peak is an atypical full convex curve. The strength of visible first crack is about 0.85 times that of flexural strength. The flexural toughness of UHPC is good, so it is better to use visible first crack deflection as the initial deformation reference to calculate the flexural toughness index.