A dual-shaft synchronous transmission fixing device has a simplified structure and minified volume. The gap between the rotary shafts can be changed or adjusted according to the transmission specification so that the electronic device can be thinned and lightweight. The fixing device includes a driver and a reactor respectively disposed on a first shaft and a second shaft and a link unit engaged with the driver and the reactor, whereby the first and second shafts can be synchronously rotated. The fixing device further includes a main fixing plate and a subsidiary fixing plate having identical structures. The main fixing plate and the subsidiary fixing plate are disposed on the first and second shafts. Each of the main fixing plate and the subsidiary fixing plate has a base section for together pivotally connecting with the link unit.