Abstract: Abstract: Porphyra yezoensis is rich in high contents of protein and polysaccharide, while that have not been fully used and developed. In order to improve resource utilization rate of Porphyra yezoensis, a new technology of alternating two-frequency countercurrent ultrasonic-assisted extraction is developed to produce high-yield protein and polysaccharide mixed products. The effects of two ultrasonic ways were investigated in this research: the composite working and the alternating working of two-frequency (two-frequency of A: 15 and B: 20 kHz, A: 15 and C: 28 kHz, and B: 20 and C: 28 kHz, respectively referred to as the AB composition, AB alternation, AC composition, AC alternation, BC composition, and BC alternation), on the product yield (expressed as the total yields of protein and polysaccharide). The response surface optimization tests for three key factors of material to water ratio, extraction time, and temperature were conducted to optimize the extraction process, based on the central composite design (CCD). Moreover, in order to analyze the feasibility of alternating two-frequency countercurrent ultrasonic-assisted extraction technique, the extraction performance of the alternating two-frequency countercurrent ultrasonic-assisted extraction method was compared with those of the mono-frequency countercurrent ultrasonic-assisted extraction and the traditional extraction methods. The results showed that the alternating ultrasonic way was significantly better than the composite one and the alternating two-frequency of 15 and 20 kHz was the most effective. The factors: material to water ratio, extraction time, and temperature had significant effects on the extraction process. Therefore, the established optimum conditions of alternating two-frequency countercurrent ultrasonic-assisted extraction were as follows: the material to water ratio was 14 mg/mL, the extraction time was 88 min, the temperature was 41oC, the pH value was 9.0, the ultrasonic power was 200 W/L, the alternating two-frequency was 15 and 20 kHz, and an alternating working time of 3 s. Under these conditions, the production capacity of protein and polysaccharide mixed products from Porphyra yezoensis achieved the highest with the product yield of (48.18?0.08)%, in which the protein yield was (24.35?0.07)%, the corresponding protein purity was (45.61?0.33)%, the polysaccharide yield was (23.83?0.02)%, and the corresponding polysaccharide purity was (44.64?0.37)%, respectively. The product yield was increased by 168% and the extraction time was decreased by 64% compared with the traditional extraction method. In addition, the product yield was increased by 50% and the extraction time was reduced by 18% compared with the mono-frequency countercurrent ultrasonic-assisted extraction. In terms of production cost, the alternating two-frequency countercurrent ultrasonic-assisted extraction is technically advantageous and is worth being promoted in the industry compared with the mono-frequency countercurrent ultrasonic-assisted extraction and the traditional extraction methods.