A method of preparing a lithium secondary battery, the method comprising steps of: preparing an electrode active material slurry; preparing a non-aqueous electrolyte solution; pressure-molding the electrode active material slurry into a form of a pellet having a top surface and a bottom surface, and having a thickness of less than 1,000 μm, a diameter of the bottom surface of 5 mm to 20 mm, a porosity of 18 vol% to 30 vol%, and a density of 1 g/cc to 4.5 g/cc; dispending 1 μl to 10 μl of the non-aqueous electrolyte solution on a top surface of the pellet and calculating an electrolyte solution impregnation rate by measuring a time at which the non-aqueous electrolyte solution passes through the pellet; and setting an estimated electrolyte solution impregnation time of a measurement target unit cell by using the electrolyte solution impregnation rate; wherein the electrolyte solution impregnation rate is calculated using Equation 1: Impregnation rate μm / sec = pellet thickness μm / time sec from a time of dispensing the electrolyte solution on the top surface of the pellet to a time of reaching a bottom surface of the pellet; and wherein the estimated electrolyte solution impregnation time of the measurement target unit cell is set by using Equation 2: Estimated unit cell impregnation time hr 0.077 hr / sec ⋅ mm 2 × electrode thickness μ m × electrode area mm 2 2 electrolyte solution impregnation rate μ m / sec wherein, in Equation 2, the estimated unit cell impregnation time denotes the estimated electrolyte solution impregnation time of the unit cell, and the electrode is a positive electrode or a negative electrode. preparing an electrode active material slurry; preparing a non-aqueous electrolyte solution; pressure-molding the electrode active material slurry into a form of a pellet having a top surface and a bottom surface, and having a thickness of less than 1,000 μm, a diameter of the bottom surface of 5 mm to 20 mm, a porosity of 18 vol% to 30 vol%, and a density of 1 g/cc to 4.5 g/cc; dispending 1 μl to 10 μl of the non-aqueous electrolyte solution on a top surface of the pellet and calculating an electrolyte solution impregnation rate by measuring a time at which the non-aqueous electrolyte solution passes through the pellet; and setting an estimated electrolyte solution impregnation time of a measurement target unit cell by using the electrolyte solution impregnation rate; wherein the electrolyte solution impregnation rate is calculated using Equation 1: Impregnation rate μm / sec = pellet thickness μm / time sec from a time of dispensing the electrolyte solution on the top surface of the pellet to a time of reaching a bottom surface of the pellet; and wherein the estimated electrolyte solution impregnation time of the measurement target unit cell is set by using Equation 2: Estimated unit cell impregnation time hr 0.077 hr / sec ⋅ mm 2 × electrode thickness μ m × electrode area mm 2 2 electrolyte solution impregnation rate μ m / sec wherein, in Equation 2, the estimated unit cell impregnation time denotes the estimated electrolyte solution impregnation time of the unit cell, and the electrode is a positive electrode or a negative electrode.