A frequency-resolved optical gating (FROG) apparatus which can be used to measure the intensity and phase of infrared long wavelength ultra-fast laser pulse in single-shot is described. Because the high damage threshold coating in this waveband is difficult and expensive, we use two half-semicircle mirrors to substitute traditional transmitted beam splitter. Meanwhile, error of measurement in the improved configuration will be diminished, and the whole device becomes more compact and easier to be adjusted. We utilized the apparatus to characterize optical parametric amplifier ultra-fast pulse pumped by Tisapphire laser. With the center-wavelength of 1.8 μm, the pulse has duration of 66.9 fs and spectral width of 54.5 nm. The group delay dispersion (GDD) in spectrum region is 202.9 fs2.