We establish a q-analogue of Sun–Zhao's congruence on harmonic sums. Based on this q-congruence and a q-series identity, we prove a congruence conjecture on sums of central q-binomial coefficients, which was recently proposed by Guo. We also deduce a q-analogue of a congruence due to Apagodu and Zeilberger from Guo's q-congruence.