<p>It was a challenge to monitor concrete structure crack under complex environmental action. To obtain conductive hydrogel material with higher stretchability, signal response sensitivity and stability to monitor concrete structure cracking, the conductive hydrogel reinforced by different content of cellulose nanocrystals, which is called polyacrylic acid-cellulose nanocrystals (PAA-CNC), were developed in this paper. The performance improvement of PAA-CNC was studied by scanning electron microscope, resistance, uniaxial tensile and cyclic tensile test. Finally, the concrete crack monitoring accuracy of PAA-CNC was verified by three-point bend loading test. The result showed that combining cellulose nanocrystals with hydroxyl in conductive hydrogels can form uniformly dispersed micelles and three-dimensional network structure, which can increase the ionic conductive path and connection strength between molecules. When cellulose nanocrystals content of hydrogel was 0.12%, the effective strain sensing range and sensitivity within the range reached the maximum. When the content of cellulose nanocrystals was 0.12, the effective strain sensing range and sensitivity of PAA-CNC will reach maximum value. Compared with other contents of cellulose nanocrystals, PAA-CNC_0.12 can produce a stable signal response when tested and quickly recover to the initial resistance after cyclic stretching. The crack width obtained by PAA-CNC_0.12 does not exceed 5% of that obtained by digital image correlation equipment.</p>