Abstract: The flotation process of molybdenum ore produces a large amount of mineral processing wastewater. To make the mineral processing wastewater meet the reuse requirements, the flocculant olyacrylamide (PAM) is usually added to accelerate the settlement of micro−grained slime so that the upper water can be reused. However, the accumulation of PAM in wastewater seriously depresses the flotation of molybdenite and reduces its recovery with the increase in the recycling times of mineral processing wastewater. The influence of PAM on molybdenite flotation behavior was investigated, and ultrasonic crushing technology was used to treat the mineral return water containing PAM. The influence rule and mechanism of the mineral return water containing PAM after ultrasonic treatment on the flotation behavior of molybdenite were investigated. The results showed that the presence of PAM greatly reduced the surface hydrophobicity of molybdenite and depressed the flotation of molybdenite, and the depression effect was enhanced with the increase of PAM mass concentration. The floatability of molybdenite recovered after PAM was treated by ultrasound. The flotation recovery of molybdenite reached 68.63% after 5 mg/L PAM solution was ultrasonic treated for 9 min at 600 W power. Further study showed that the micro−gas nucleated cavitation bubbles in solution vibrate under ultrasonic, grow and collapse under a certain sound pressure. It is accompanied by the production of free radicals, which can crush and oxidize and degrade PAM, and the molecular weight of PAM decreased rapidly. Treatment of PAM by ultrasonic crushing technology could improve the reuse rate of molybdenum ore dressing wastewater, the flotation recovery of molybdenite, and the recycling efficiency of water resources.