Alkali-activated fly ash （AAFA） binder was prepared with ultra-fine fly ash partially replacing Class Ⅰ fly ash at room temperature， and its flowability was optimized by superplasticizer. The results show that the initial setting time of AAFA paste with 50% ultra-fine fly ash is 42 min， and the 3 d compressive strength is 12.78 MPa. As the apparent viscosity is too large to make it possible to test its fluidity value in conventional tests， naphthalene superplasticizer is added to enhance the dispersion of fly ash particles in AAFA paste， with the result that it significantly reduced the apparent viscosity， and thus improves the initial fluidity and fluidity retention of the paste. However， as the fly ash particles are encapsulated and the connection between the gel products is weakened， the setting time of the AAFA paste is prolonged so as to be unfavorable to the compressive strength of the early age. Naphthalene-melamine composite superplasticizer also significantly reduced the initial apparent viscosity of AAFA paste and improved its initial fluidity. Moreover， due to its influence on the polymerization process， the development of compressive strength of AAFA paste is promoted， while the fluidity loss is accelerated.