4-20mA current loop design
As Feilthry pointed out my problem might be commom-mode chokes L4 and L5. So, I tried replacing them with new ones (new part is RL822-3R3K-RC), then my problem was solved. It now can drive 4-20mA current consistantly on load resister at 0-500ohms.  

Sorry that I can't find a datasheet for KV03405 to compare parameters. I think it's because the new chokes got a highter Self Resonant frequency than last one.