<p>摘要：We mainly introduce the spectral analysis which is very different from the classical AKNS type Lax pair, such as NLS\MKdV equation, since the short pulse equation admits a WKI type negative order Lax pair. We show the solution of the initial value problem for the short pulse can be reconstructd in terms of the solution of a 2*2 matrix Riemann-Hilbert problem. Then, using the nonlinear steepest descent or Deift-Zhou method, we can obtain the leading order asymptotic behavior as time goes to infinity under no solitons assumption. And this result is more accurate than the result obtained by PDE method, because of the complete integrable property of the short pulse equation. We also show that the no solitons assumption is possible under some special initial value.</p>