The article shows the results of floristic and geobotanical vegetation investigations (flora, vegetation) in the complex state natural zakaznik (refuge) of the regional importance «Big Tassor Lake», created in 1999 in order to maintain the ecological balance, conservation of outcropping variegated clays and pristine feather grass steppes areas. The territories due to which it is necessary to carry out the expansion of the refuge have been identified. It is proposed to increase the area of the refuge «Big Tassor Lake» due to the near-border non-arable and fallow lands from the current border of the refuge to the border of the Kazakhstan Republic, including Small Tassor Lake, the watershed lake Dongoleksor and the Russian part of the coast lake Chinkursor. This expansion will increase the territory of this protected area up to 3919 ha. The necessity to expand the refuge is required by the low representation of the South Kulunda plant communities including alkali-saline complex, real feather grass and fescue steppes enriched by the desert species, shrub steppe communities, including karagana shrub.