This is a technical description about preparing high-strength and water-resistant phosphorus gypsum-based sandless self-leveling mortar by using phosphorus building gypsum, P·O 42.5 cement, fly ash, mineral powder, stone powder, and additives as raw materials. The optimal dosage of cementitious materials in mortar was determined through orthogonal experiments. The effects of water reducer and redispersible latex powder on mortar performance were studied, and microscopic analyses of mortar were conducted using XRD and SEM. The results indicate that when the mass ratio of phosphorus building gypsum, cement, fly ash, mineral powder, and stone powder is 73∶5∶5∶15∶2, the mortar shows optimal comprehensive performance. The 28 d absolutely dry compressive strength of mortar reaches 33.0 MPa, with a softening coefficient of 0.774. Water reducer can enhance the liquidity, mechanical properties, and water resistance of mortar within 30 min. However, at a dosage of 0.30% (mass fraction), the water reducer can decrease the mortar’s long-term strength. Redispersible latex powder decreases the liquidity and mechanical properties of mortar, but improves its water resistance. The phosphorus gypsum-based sandless self-leveling mortar prepared in this paper meets the requirements of "Gypsum-based self-leveling mortar" (JC/T 1023—2021). The 28 d absolutely dry flexural strength and 28 d absolutely dry compressive strength of mortar are 12.0 and 45.9 MPa, respectively, with a high softening coefficient of 0.886 and a low water absorption rate of 2.8%.