The invention discloses a method for producing ultra-low phosphorous steel through single slag of a converter. According to the method, the content of Al2O3 in final slag is controlled to be 9%-11% by adjusting a feeding mode at the earlier stage of smelting, the melting point of slag is made to be at the temperature of about 1480 DEG C, and accordingly the mobility of the slag is improved, and the slag is promoted to be molten early and fully; good mobility of the slag can be guaranteed, and the dephosphorization effect at the earlier stage can also be improved; by controlling the adding amount and the adding mode of cold materials during converting, the temperature rising speed at the earlier stage of converting is controlled, low-temperature dephosphorization time is prolonged, and the dephosphorization efficiency of the converter is improved; the endpoint temperature of molten steel is controlled to be within 1640 DEG C, and steel is discharged at the low temperature; an appropriate amount of limestone is added when the temperature is higher than 1640 DEG C, so that the temperature of a welding pool is reduced, the slag stirring effect is improved, high-temperature rephosphorization is prevented, and it is guaranteed that the endpoint phosphorus content is within 0.008%; efficient, environment-friendly, stable, low-cost and high-controllability production of ultra-low phosphorous steel through the existing converter is achieved.