Vacuum membrane distillation (VMD) is considered an effective method for concentration of herbal aqueous solutions (HAS). However, membrane fouling and flux decline are complex problems in such concentration processes. The concept of threshold flux was proposed for optimizing the membrane process. Whether it is useful for guiding optimization of the VMD process on HAS concentration needs further investigation. The Semen Raphani aqueous solution (SRAS) was taken as an example to investigate the membrane flux behavior during SRAS concentration by VMD. The results confirmed that the threshold flux concept is applicable to concentrate HAS by VDM. At or below the threshold flux, the total resistance was independent of the flux, while above it membrane fouling increased with higher membrane flux. Moreover, limiting the flux should be avoided in the membrane process because of strong fouling and flux decline.