The invention discloses a preparation method of an alkali-free and chloride-free liquid accelerator. The liquid accelerator is prepared from the following raw materials: 30%-55% of aluminum sulfate, 6%-8% of sodium citrate, 5%-14% of a polycarboxylate superplasticizer, 3%-15% of a modifier, 0.5%-5% of a stabilizer, 0.1%-1% of a thickener phosphoric acid for adjusting the pH value to 4-6 and the balance of water. When the mixing amount of the accelerator prepared by the method is 4%-6%, the initial setting time is 1.5-3.0 minutes; the final setting time is 5.0-6.0 minutes; the compressive strength of 1d is greater than 10MPa; and compared with a blank mortar block, the compressive strength rate of 28d reaches 100%. Compared with a conventional accelerator, setting and hardening of cement mortar can be effectively accelerated; and the accelerator prepared by the method has good adaptability to cement, high long-term strength, stable system property, low dust concentration and practical application value.