Now the problem is that the GPU needs 400W power supply. But in my desktop I only got 300W. But the amd a8-6500 doesn't even use 100W (65W). Can I still be able to put this gpu in and make it work normaly?
You would be better off upgrading your PSU. They're not saying that the GPU draws 400W by itself, but that in an average system the GPU with the other hardware usually draws close to 400W, so they recommend it as a minimum to maintain a safe margin.
Also, since you have a prebuilt system, not only is that PSU only chosen to provide enough power to the system as originally bought, but the OEM saves money by picking a cheaper, lower-quality PSU in the first place. Since you're adding a discrete GPU, the chances of the 300W low-quality PSU being able to reliably provide power for an additional discrete GPU that recommends a higher-rated PSU are going to be pretty much nil. If you're lucky, it'll just refuse to POST, or just show a black screen (no video). if you're not so lucky, you might actually damage some components.
Considering that you can pick up an 80+ Bronze Certified 500-550W PSU for under $60, spring for the PSU upgrade to go with your GPU.