We study the formation mechanism of the double piral arms in vortex with the eddy equation in column coordinates. Results show that growth of undulations is very quick when m=2（double spiral arms） in basic flow field,whereas the increment rate is limited over time. Undulations can evolve into double piral arms and expand toward outside. Mechanism of undulation development is the strong advection process in which vorticity is transfered from basic to undulation field.