Propylene epoxidation catalyzed by titanium silicalite is easily influenced by acidic or basic impurities. The effect of sodium ions on the reaction was systematically investigated. It has been shown that a proper amount of sodium salt additive can inhibit the further reaction of propylene oxide (PO) with the solvent and can increase the selectivity of PO. But a slightly excessive amount of sodium salt can be a poison to the titanium silicalite catalyst. The sodium salt additive can accumulate on the catalyst in repeated runs of propylene epoxidation in an autoclave, so the catalyst may be poisoned if an equal amount of sodium salt is added in each run. The excessive sodium ions on titanium silicalite can deactivate the catalyst in calcination, while acid treating can restore the activity of titanium silicalite. If the propylene epoxidation is carried out in a fixed bed, the sodium ions on titanium silicalite can gradually come loose and the PO selectivity decreases with time on-stream. A small amount of supplement of sodium salt in the raw material can make the PO selectivity stable for a longer period.