We demonstrate experimentally a low switching energy and high-linearity all-optical sampler based on terahertz optical asymmetric demultiplexer (TOAD) composed of a nonlinear semiconductor optical amplifier (SOA) with a multiple quantum well structure. Effects of the sampling pulse power and asymmetric offset of SOA on the shape, width and amplitude of sampling windows are analyzed in detail respectively. It is found that the sampling pulse power has no effect on both the shape and the width of sampling windows, but has a significant effect on the window amplitude. Meanwhile there exists an optimal power which maximizes the sampled output and determines the switching energy of TOAD. The asymmetric offset of SOA from the center position in the loop determines the width of sampling windows and has great influences on both the shape and the amplitude of the sampling window. The sampling windows with different widths have approximately the same rise edge due to the fast response of SOA for the sampling pulse. However, the normalized amplitude of sampling windows firstly increases sharply with the increase of the asymmetry, then gradually flattens out, and tends to be stable in the end. In addition, the switching energy and linearity of TOAD are studied. The switching energy is as low as 25 fJ, and the linearity is as high as 0.99. Moreover, at different window widths, the switching energy of TOAD remains the same and the sampling windows have a very good linearity. However, the sensitivity of a TOAD sampler with different width is different: the wider the sampling window, the higher the sensitivity and the larger the corresponding dynamic range.