How can I route a single complicated side PCB design ? Specially on Easyeda?
I do not think you can do this with a single sided board. 

If it is at all possible on a single sided board, then you will have to use jumpers.

A jumper is a piece of wire that you solder in place like a resistor.  It allows you to "jump over" (hence the name) other traces.

So, you would route as many traces as possible.

Then you see if you can route the remaining traces by using jumpers.

If you can do it at all, you will need a lot of jumpers on a single sided board.

If you could switch to a double sided board, it will be much easier to route - though I think you will probably need a few jumpers even on a double sided board.

As mentioned in the comments, a doubled sided board costs about the same as a single sided if you have it manufactured.



If you make it yourself at home, then you will probably have to stick with single sided.  You will have a lot of holes to drill - for each IC pin and connector and every jumper.

The solder pads will have to be a bit larger than usual to allow you to drill the holes.

You will need very small drill bits, and a drill that rotates smoothly.  A drill press like you would use for metal or wood working with typical drill bits (1mm and larger) typically wobbles a good bit - often times, the wobble will be as large as the diameter of the drill bits you would use to drill holes in a PCB. That would cause the drill press to break the bits.

You will also need to make the board somewhat larger to have space for the jumpers.



A board with jumpers looks like this:



Image borrowed from this other question. 



As for how to accomplish a single sided board with jumpers, I think Peter Bennett has the best idea:

Route the board double sided, and arrange all the top side traces to be short, in groups, and in easy to reach spaces.

That automatically sets the vias and the drill holes where you need them.

Basically, you route a double sided board, but don't print or etch the top side.

All of the top side traces are replaced with wires.

With a little care, you can make it look nice and neat - and, more importantly, be easy to finish when soldering in your short wires on the top side.  Neat doesn't just look good.  It also helps with "easy to build."