To explore the effects of particle formation environments on gene carrier-DNA particle size and gene delivery efficiency, the particle size, binding affinity were measured of BDCP (cationic polymer, having PEI 600 backbone with biodegradable bond) in varied ionic conditions(PBS, 5% glucose, H2O). And the gene delivery efficiency and cytotoxicity were tested in similar conditions. Results showed that PBS was favorable to achieve improved stability, high gene delivery efficiency and low hemolysis effect, and BDCP/plasmid combinded in 5% glucose or H2O were found weaker binding affinity and lower transfection efficiency. Cationic carrier/DNA particle size, affinity, and gene transfection efficiency are affected by ionic strength.