The invention discloses a method for generating macrogametocytes by somatic cells of inducing undaria pinnatifida parthenogenesis juvenile sporophytes, which has easy operation, low cost and high maturation rate of the generated macrogametocytes and can provide a large quantity of the macrogametocytes for the large-scale seedling production of undaria pinnatifida. The method comprises the following steps of: cutting undaria pinnatifida macrogametocytes stored and cultured in a room into pieces and then culturing under the conditions of temperature suitable for the undaria pinnatifida macrogametocytes to grow to be mature, illumination and nutritive salt, wherein when the undaria pinnatifida macrogametocytes grow to be mature, form oocysts in large quantities and ovulate with ovums, a small quantity of the fertilized ovums are germinated to seedlings through parthenogenesis because unfertilized ovums are died by being unfertilized by spermatozoa; and when the somatic cells of the parthenogenesis seedlings are grown to 0.4-0.6 cm, adopting a rhizoid cutting method to enable fronds to lose polarity and constraint on the somatic cells, and inducing the somatic cells of the seedlings to generate the macrogametocytes.