In order to solve the problems in the alginate production process of low-viscosity and ultra-low-viscosity, in this study, we degraded Laminaria japonica (cultivated in Qingdao, Shandong) with align lyase to produce low-viscosity and ultra-low-viscosity aliginate and investigated the effects of molecular weight, pH and temperature on viscosity, then the optimum conditions for alkali digestion were determined. On this basis, we studied the effects of enzyme dosage, enzymolysis time and initial viscosity of raw materials on alginate products. The results showed that the low-viscosity and ultra-low-viscosity alginate can be obtained within 30 min. of enzymatic hydrolysis when the amount of enzyme was controlled within the range of 100~500 U/g, which the low-viscosity alginate can be obtained when the amount of enzyme was 100~330 U/g and the ultra-low-viscosity alginate can be obtained when the amount of enzyme was increased to 330~500 U/g. Meanwhile we found that the enzymatic hydrolysis of alginate samples was with high uniformity of molecular weight, and the water saving rate was as high as 10%~50% during the technology process. At the same time, it was found that the viscosity of the hydrolyzed sample was not correlated with the initial viscosity of the raw material, so the process had extensive applicability of the raw materials.