Abstract: Low molecular weight fucoidan (LMWF) was experimentally confirmed to be readily absorbed through oral administration and was scientifically validated to exhibit multifunctional biological activities, particularly hypoglycemic effects, as evidenced by its therapeutic efficacy in diabetic nephropathy and atherosclerosis management. In this study, the inhibitory effects of LMWF on α-amylase and α-glucosidase were systematically investigated through enzymatic reaction kinetic assays, and Lineweaver-Burk double reciprocal plots were generated to elucidate the inhibition mechanism of LMWF on these enzymes. LMWF was applied in the development of low glycemic index (GI) food products, in which the optimal formulation ratio and critical processing parameters were systematically determined through single-factor experiments and orthogonal array testing, thereby establishing an innovative pathway for the functional utilization of marine-derived LMWF in diabetic-friendly food preservation and nutritional enhancement applications. Results demonstrated that UF (Fucoidan from Undaria pinnatifida Suringar) and EUF obtained by enzymatic degradation of UF (Enzymatic degradation of UF, EUF) exerted significant inhibitory effects on both α-amylase and α-glucosidase, and the effects of EUF were observed to be stronger than that of UF, which indicated that the activity of fucoidan was enhanced after degradation. The IC₅₀ of EUF on α-amylase and α-glucosidase were determined as 9.188, 1.118 mg/mL, respectively, suggesting that the inhibitory ability of EUF on α-glucosidase was significantly stronger than that on α-amylase. The inhibitory type of EUF on α-amylase was identified as anticompetitive inhibition in reversible inhibition, and the inhibitory type on α-glucosidase was characterized as a mixture of competitive and noncompetitive inhibition in reversible inhibition. The optimal formula of low-GI milk powder was established as: skim milk powder:whole milk powder=1:0, the addition of inulin was 0.4%, EUF was 0.005%, isomaltulose was 0.7%, in combination with homogenization pressure of 500 Pa. The formulated milk powder exhibited a GI value of 28.12 along with favorable sensory properties and high nutritional value.