The invention relates to a continuous extrusion process for preparing cold water swelling phosphate-cross-linked starch derivatives, wherein the process consists of the subsequent steps of gelatinising an alkaline starch in a first zone (10) in the extruder; adding of POCl3 as cross-linking reagent in a second zone (11) in the extruder; cross-linking the obtained gelatinised starch by means of the POCl3 in the second and third zone (11, 12) in the extruder, and neutralising the obtained cross-linked gelatinised starch that is present in a fourth zone (13) in the extruder; recovering the obtained cross-linked, gelatinised starch in a fifth zone (14) in the extruder.