The invention discloses a preparation method of tubular starch derivative, and belongs to the technical field of modified starch processing. The invention relates to tubular starch derivative, which has a narrow polymerization degree range and is prepared through combination of ultrasonic de-cluster starch, isoamylase debranching, and alcohol step precipitation fractionation technologies. The method comprises the main steps that waxy rice starch is selected as a raw material, and then tubular starch derivative products, with the polymerization degree (DP) being 20 to 32, are obtained through gelatinization, ultrasonic de-cluster (ultrasonic processing is performed for 3s, and then ultrasonic processing is stopped for 3s, the total time is 15 to 20min, the frequency of ultrasonic wave used is 20kHz, and the power is 200W), isoamylase debranching (the reaction pH is 3.5 to 5.0, the reaction temperature is 37 DEG C, and the reaction time is 12 to 16h), alcohol step precipitation fractionation and drying. The method has the advantages of simple processes, low cost, narrow polymerization degree range of the tubular starch derivative products obtained, good embedment characteristic, stable quality and high solubility.