Abstarct: In recent years, water deficit in Iran, especially in arid and semi-arid regions, has necessitated the need to replace sugar beet plantation with plants with higher water use efficiency. Sweet sorghum is one of the sugar plants with low water requirements and high environmental compatibility that provides economical sugar production in different regions. However, the physiological aspects affect sugar yield in this plant is not fully understood. The present experiment examines the effect of sink removal of as a limiting factor for sugar production and magnesium spraying as a factor in improving plant growth on photosynthetic pigments and yield of two sweet sorghum varieties. This research was carried out in a factorial baxsed on a randomized complete block design with three replications in two years (2014, 2015) in Shahroud Agricultural Research Center. Experimental factors included sweet sorghum cultivars (KFS2 and KFS3); sink removal treatments (control, mechanical removal and chemical removal using ethephon) and spraying different concentrations of magnesium (0, 4 and 8 mM). The results of this study showed that in different years of experiment, KFS3 had significant differences with KFS2 for plant height, stem diameter, fresh forage yield and water use efficiency in terms of dry matter. Among sink removal treatments, mechanical and chemical removal had the greatest effect on the measured traits. In addition to increasing biological yield and photosynthetic pigments, these treatments increased the amount of sucrose and total sugars in syrup compared with intact plant and zero level of magnesium spraying in combination with the concentrations of 4 and 8 mM magnesium. Due to the no significant difference in most traits between 4 and 8 mM levels, a 4 mM level of magnesium is recommended. The highest yield of sugar was 4166 kg ha-1, and the highest water use efficiency was obtained in terms of sugar yield 0.75 kg m-3 from chemical sink removal and 4 mM magnesium in KFS2. In two varieties, assimilate surface index was significantly affected by sink removal and the highest rate obtained from chemical removal with amount of 2.65 g m-2 of leaf area, which, along with other traits indicates that the photosynthetic capacity of the plant is enhanced by sink removal, especially by chemical removal with ethephon. In general, with the implementation of this research, it can be concluded that using the ethephon as a substitute for mechanical removal of the sink removal by preventing flowering in sweet sorghum, the quantitative and qualitative growth of the cultivars of this plant could be increased. Also, the use of different concentrations of magnesium in addition to this combination increased sugar yield.