1. I Am learning cmos image sensor. and want to design a sensor,
but I don't know what software is much easy to use.
You can use any software as long as you have the right layers to do the layout... since it seems you're not using a specific process with its own PDK you will basically have to start from scratch, so you won't probably have files to simulate, run DRC, LVS etc.
2. The sensor have many pixels, so when layout , just copy a pixel and pasete to form the array, is this right?
Yes
and the schematic also need draw many photodiode, so do the LVS, is this right?
Yes
3. I find a opensource software, name electric CAD, Can we use this software to design CMOS image sensor?
Yes, but as I said you have to do everything on your own... which is quite a complex process
diemilio