Abstract: Dwarf and semi-dwarf varieties of summer squashes are two domestic types in China and semi-dwarf varieties are mainly cultivated under greenhouse conditions. To study the effect of density on canopy and source-sink relationship in summer squash, key high-yield factors of summer squash were tested in large plastic houses by using two semi-dwarf varieties ("Donghu No.4" and "Dongyu") and two dwarf varieties ("Changqingwang No.3" and "Zaoqing"). The results showed that as planting densities increased from low to high, the changes in canopy indicators of semi-dwarf varieties were far greater than those in dwarf varieties. The trends of change in LAI (leaf area index), source capacity and sink capacity tracked quadratic-type curves. Transmittance had an inverse trend of change with LAI. The trend of change in source-sink ratio was linear and the highest yield was at optimum source-sink ratio. Lower LAI and higher light transmittance, and lower source supply, sink capacity and source-sink ratio were noted under lower density. Lower summer squash yield was associated with lower sink capacity under lower density. Light transmittance for all tested varieties was lower under medium density treatment with mean values of 12.4% for semi-dwarf and 13.2% for dwarf varieties. However, LAI was higher under medium density treatment with maximum LAI of 4.4 for semi-dwarf and 3.3 for dwarf varieties. The higher yield under medium density was associated with higher capacities of source and sink and source-sink ratio. The maximum source supply was 1 169.8 g·m^-2 for semi-dwarf varieties and 736.9 g·m^-2 for dwarf varieties. Also the maximum sink capacity was 422.4 g·m^-2 for semi-dwarf and 333.0 g·m^-2 for dwarf vine. Then the almost optimal source-sink ratio was 2.62 for semi-dwarf and 1.96 for dwarf varieties. Summer squash LAI under high density treatment increased rapidly at early growth stage with lower light transmittance especially at maximum LAI. However, LAI decreased rapidly due to rapid senescence of lower position leaves at late stage, resulting in larger light transmittance. Under high density, source supply was the dominant limiting factor of yield. Semi-dwarf varieties yield was higher than dwarf yield varieties because of higher and more stable source supply. The largest source supply capacity of semi-dwarf variety was 1.6 times that of dwarf variety. It was concluded that semi-dwarf variety of summer squash was a better choice for protected fields.