Abstract: In order to achieve sustainable extraction and enhance the properties of astaxanthin from Haematococcus pluvialis, a green and efficient approach was developed by utilizing Tween-20 under ultrasonic conditions. This approach allowed for the efficient release of astaxanthin while simultaneously preparing water-dispersible astaxanthin emulsions. The factors influencing the process, the formation mechanism of the water-dispersible astaxanthin emulsion, and the composition and performance characterization of the obtained product were investigated. Exploration of the process revealed that Tween-20 dosage, ultrasonic power, and solid-to-liquid ratio significantly influenced the release and dispersion efficiency of astaxanthin, with 98.41% astaxanthin being released and dispersed when Tween-20 dosage, ultrasonic power, and solid-to-liquid ratio were 200 μL, 600 W, and 1:20 g/mL, respectively. The mechanism exploration results demonstrated that ultrasound had a dual effect of disrupting the cells of H. pluvialis to a limited extent and assisting Tween-20 to disperse astaxanthin, which was superior to high-pressure homogenization. The characterization results showed that the water-dispersible astaxanthin emulsion obtained under these conditions had an average particle size of 115.55 nm, a Zeta potential of −23.35 mV, and an encapsulated astaxanthin content of up to 43.82% in the dry product. Astaxanthin was found to be encapsulated in an amorphous non-crystalline state, with selective encapsulation of saturated fatty acids. The amount of saturated fatty acids was up to 17.93% (wt.%). Overall, this one-step, ultrasonic approach to preparing water-dispersible astaxanthin emulsions from H. pluvialis using Tween-20 is easy to operate, highly efficient, green and feasible, with some of the product properties being superior to those of pure astaxanthin encapsulated products.