Thanks for everyone!
After many times experiments ,this problem had been solved.I found that if I had add a command(set_ultra_optimization false)in the synthesis script,the synthesis result would be the sane as our expectation and the reset signal is only connectted to the AS pins of Flip-flops.Through looking up the userguide of DC,I find out that the ultra_optimaztion is enabled by default. So if we want to disable this function of DC ,we must disable it expllicitly in command line.The ultra_optimization function of DC is a powerful about how to optimize the datapath by advanced method and the synthesis result of it is indeed better than DC expert synthesis,but I don't know how can it generate such result and let the reset signal join in the datapath?
Fortunately,we get a right netlist ,although the remain question is still needed to be solved!
Best regards to everyone!