Aiming at the problems that a large amount of slag sludge is difficult to be treated in the excavation of urban pipe network and the backfilling of the basement is lacking in good quality backfilling, the technology of improving the slag sludge to the flowable fill for in-situ use is studied and applied in a demonstration project. Measurements of the fluidity and the strength by indoor experiments indicates that the fluidity increases significantly with the increase of water consumption, and the strength increases with the increase of the amount of solidified material and the age of curing. The water-solid ratio and the lime-sand ratio can be used as the ratio index parameters of the fluidized backfill. When the water-solid ratio is 0. 63 and the lime-sand ratio is 0. 15, the flowable fill has a fluidity value of 242 mm, and it has the early-strength performance, which can reach 98 kPa in one day and 390 kPa in 28 days, meeting the design of fluidized backfill. This paper discusses the application and effect of this technology in engineering in combination with the pipe network embedding project in the management of Maozhou River in Dongguan. Through the on-site construction and the follow-up inspection, it shows that the slag excavated on site is mixed with the above ratio, the flow value is between 188-218 mm, and the strength at 28 d is between 324-362 kPa, fully meeting the requirements of pipe network backfill.