  In this paper, based on the overlapping domain decomposition method (DDM)
proposed in \cite{Leng2015}, an one step preconditioner is proposed to solve 2D
high frequency Helmholtz equation. The computation domain is decomposed in both
$x$ and $y$ directions, and the local solution on each subdomain is updated
simultaneously in one iteration, thus there is no sweeping along certain
directions. In these ways, the overlapping DDM is similar to the popular DDM
for Poisson problem. The one step preconditioner simply take the restricted
source on each subdomain, solve the local problems and summarize the local
solutions on all subdomains including their PML area. The complexity of solving
the problem with the preconditioner is $O(N n_{\text{iter}})$, where
$n_{\text{iter}}$ is the number of iteration, and it is shown numerically that
$n_{\text{iter}}$ is proportional to the number of subdomains in one direction.
2D Helmholtz problem with nearly a billion unknowns are solved efficiently with
the preconditioner on massively parallel machines.
