Abstract: In order to solve the problem of insufficient research on the nutritional properties and digestibility of flour streams in wheat milling systems, the particle size and microstructure of 31 flour streams in different systems were analyzed and determined by Malvern laser particle size analyzer, scanning electron microscope and confocal laser scanning microscope, and the differences of starch and protein digestion properties were explored by in vitro digestibility simulation methods. The results showed that the moisture content of the system flour decreased and the protein and wet gluten content were on the rise with the backward movement of the milling flow in the milling system. The ash content of break flour (BF), reduction flour (MF) of the posterior flow and tailings flour (TF) were higher than those of other system flour. The starch and damaged starch content of BF were lower than those of MF overall. Compared with BF, scratch flour (SF) and TF, the particle size distribution of MF was more uneven. The endosperm in MF was gradually extruded into sheets. Due to the shear effect, the block endosperm with neat crosssections appeared in BF, while the endosperm block of SF and TF showed more complex microstructures. Compared with other system flour, the content of rapidly digestible starch (RDS) in MF was generally higher and the content of slowly digestible starch (SDS) in MF fluctuated greatly. The system flour in previous milling flow has higher content of anti-digestible starch (RS). The starch digestibility of MF and TF was higher than that of BF and SF as a whole. Compared with MF and TF, the protein digestibility of BF and SF was relatively higher. In general, the digestibility of BF starch is low, while the protein content and digestibility are relatively high. Therefore, compared with other system flour, BF shows higher sugar control ability and protein nutritional value.