Low molecular weight fucoidan (LMWF) has been proved to be more potent than its prototype, many degradation methods have been used to prepare LMWF. This study is conducted to further explore the biological activities of LMWF prepared by ultrasound based on anticoagulation, antioxidation, and inhibition of urate induced pyroptosis and reabsorption transporters overexpression in human renal tubular epithelial cells. Data revealed that ultrasound successfully degraded fucoidan to be LMWF, the product treated for no more than 2.5 h has stronger in vitro anticoagulant activity that reached the peak value at 2.5 h demonstrated by notably increased activated partial thromboplastin time (APTT) and prothrombin time (PT), but antioxidation progressively reduced along with treatment time indicated by decreased scavenging capacity of DPPH and hydroxyl radicals. LMWF with higher content of sulfate acid group possesses stronger inhibitory activity of urate-induced overexpression of urate-reabsorption transporters and pyroptosis in human renal tubular epithelial cells than native fucoidan. Degradation kinetics model of fucoidan exposed to ultrasonic treatment was best fitted to the second-order kinetic equation. These results indicate that LMWF is more effective for renal protection and anticoagulation than high molecular weight fucoidan, suggesting LMWF is a more promising marine drug.