I just open a PNG image and save it again, but Photoshop make my image low quality. How can I solve this problem?
Original pic :
You need to save you image as a 32 bit PNG. When saving your PNG from Photoshop, make sure you select 'PNG-24' with transparency checked (the missing 8 bits from the 32 bit PNG you need).
PNG-8 (in Photoshop) only supports 1-bit transparency, which means every pixel is either transparent or not, which is why you get the jagged edges.
According to this post on SuperUser you can save a PNG-8 with full alpha transparency if you use Export instead of Save for Web, but I don't have CC to check. In CS6 your stuck with using PNG-24 or the 1-bit transparency.