Abstract: The dynamic compression stress-strain relations of iron specimens with three sizes were obtained with a split Hopkinson pressure bar apparatus. Based on these experimental results, an empirical model was presented to quantitatively describe the influence of bar/specimen contact surfaces indentation on strain measurement in SHPB experiments. This research shows that the influence on elastic strain is obvious, even when the uneven stress states in specimens eliminated by using pulse shapers; while the influence on plastic strain is related to the hardening modulus and length of the specimen: the influence will be obvious when the hardening modulus is large and the size of the specimen is comparatively small, and the correction for plastic strain is needed.