The invention provides a yarn dyeing process which includes the steps: firstly, placing yarns on a loosening cylinder to loosen the yarns; secondly, adding the yarns into alkaline liquor to whiten theyarns, and adding stabilizers; thirdly, adding acid to neutralize the whitened yarns, and washing the neutralized yarns; fourthly, stewing the washed yarns at high temperature; fifthly, primarily dyeing the yarns stewed at high temperature; sixthly, steaming the primarily dyed yarns; seventhly, secondarily dyeing the steamed yarns; eighthly, adding the secondarily dyed yarns into a color fixing agent to fix colors of the yarns; ninthly, dewatering and drying the yarns, and baking the yarns. According to the method, primary dye fixation of the yarns is implemented in a steaming manner after primarily dyeing of the yarns is implemented, secondary dye fixation of the yarns is implemented in a baking manner after secondary dyeing of the yarns is implemented, fastness of dyed colors of the yarns is improved, the color of the yarns cannot be easily faded, and the service lives of the yarns are prolonged.