The invention discloses a hydrothermal synthesis method for sodium niobate powder, which comprises the following steps: putting niobium pentaoxide and sodium hydroxide in a molar ratio of 0.05-10 into an autoclave, and adding deionized water into the autoclave according to a filling rate between 50 and 90 percent to adjust the concentration of the sodium hydroxide to between 0.1 and 15 mol / L; sealing the autoclave, putting the autoclave into a pit furnace or an oven, raising the temperature to between 120 and 240 DEG C in a temperature rise rate of 1 to 5 DEG C per minute, preserving heat for 4 to 72 hours, and then cooling along with the furnace; and taking out and opening the autoclave, pouring an obtained product into a beaker, repeatedly filtering and washing with deionized water until the washed solution is neutral, putting obtained powder into an oven, and baking at a temperature of between 40 and 100 DEG C for 6 to 24 hours. The method is simple, saves energy, reduces emission, has low cost, and is suitable for mass production; and the prepared sodium niobate powder has high purity, good fluidity, narrow particle size distribution, low degree of particle agglomeration, and perfect crystal growth.