The present disclosure provides an electrolyte and an electrochemical energy storage device. The electrolyte comprises an electrolyte salt and an additive. The additive comprises a sulfonic ester cyclic quaternary ammonium salt and a dioxide heterocyclic compound. Under the combined effect of the sulfonic ester cyclic quaternary ammonium salt and the dioxide heterocyclic compound, a dense, uniform and stable passive film on a surface of each of the positive electrode film and the negative electrode film of the electrochemical energy storage device, thereby reducing the contact between the positive, negative electrode active materials and the electrolyte, thus avoiding the occurrence of continuous oxidation and reduction reaction of the electrolyte on the surface of each of the positive electrode film and the negative electrode film, so as to make the electrochemical energy storage device has excellent high temperature storage performance and high temperature cycle performance.