The invention relates to an alkali-free chloride-free liquid accelerating agent as well as a preparation method thereof and application thereof. The liquid accelerating agent is an aqueous solution comprising 5-15 parts of a poly(aluminum carboxylate) liquid dispersing agent and 30-50 parts of aluminum sulfate. The liquid accelerating agent can be widely applied to tunnels, maritime work, support of subway, repairing, leakage prevention, water sealing and the like. Concrete using the accelerating agent is remarkable in accelerating effect, is low in rebound ratio and is high in early strength. The alkali-free chloride-free liquid accelerating agent is non-toxic and harmless, is low in comprehensive cost, and is free of pollution during production, transportation and use.