In a method for the continuous precipitation of lignin from black liquor black liquor is provided so as to flow as a pressurized flow in a reactor with a dwell time of less than 300 s. An acidifying agent selected from the group of carbon dioxide, acid and their combinations is led to the flow at one or more feeding sites to lower the pH of black liquor. The pH is allowed to decrease by the effect of the acidifying agent in the pressurized flow to the precipitation point of lignin, the pressure of the pressurized flow is abruptly released, and lignin particles are separated from black liquor.