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 fluorocarbon surfactant. Under the combined effect of the sulfonic ester cyclic quaternary ammonium salt and the fluorocarbon surfactant, a dense, uniform and stable passive film can form on a surface of each of a positive electrode film and a negative electrode film of the electrochemical energy storage device, and infiltration capability of the electrolyte on each of the passive film of the positive electrode film and the negative electrode film can be increased at the same time, so as to decrease electrode polarization inside the electrochemical energy storage device and make the electrochemical energy storage device have excellent rate capability, low temperature discharge performance and high temperature storage performance.