SRX

 ip-monitoring {
        global-weight 255;
        global-threshold 50;
        family {
            inet {
                2.2.2.2 weight 50;
            }
        }
    }
}

config is as above

1:I want to know ,ping check is only sent from primary node to 2,2,2,2 ,right? secondary node doesn't do anything at this point,right?

2:I tried to use monitor traffic interface reth2.0(suppose it is 2.0 which send icmp check) to check the detect,but no trffic is shown with respect to 2,2,2,.2

3:if primary node failed to ping 2.2.2.2,it will failover to secondary node,but in case the other node can't ping 2.2.2.2 ,will it failover and failback again and again?

1. The box will ping 2.2.2.2 from whichever interface has the best route to it - you need to think of both nodes as a single box with lots of interfaces.  If you reach 2.2.2.2 via a reth, then the ping will physically travel along the link attached to the primary node.

2. Does it have a route in the global table via reth2.0? eg:

dfex@gw240# run show route 2.2.2.2  

: 31 destinations, 34 routes (31 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

2.2.2.0/28      *[Static/5] 3d 01:49:18
                    > 

 3. Only if you have configured pre-empt on the redundancy-group that the reth2.0 interface is a member of, otherwise is will stay failed-over even when 2.2.2.2 becomes reachable again.