The invention discloses an automatic in-car air purifying system of a car. The automatic in-car air purifying system of the car is convenient to control and good in purifying effect. The automatic in-car air purifying system of the car comprises an ECU and an air conditioner system; the ECU comprises an MCU and a PM2.5 sensor; the air conditioner system and the PM2.5 sensor are both connected with the MCU; the air conditioner system comprises an air blower, a compressor, a condenser, a dry reservoir, an evaporator, an air conditioner controller, an air conditioner control panel and a mixed air adjusting valve; the air blower is arranged on an air inlet of the air conditioner system; the compressor is connected with the condenser connected with the dry reservoir; the dry reservoir is connected with the evaporator; a filter is arranged between the air blower and the evaporator and comprises a framework, an outer cover, a primary filter element and a secondary filter element; the outer cover, the primary filter element and the second filter element are sequentially arranged inside the framework in the air flowing direction; flow deflectors are arranged on the inner surface of the outer cover, are in a fan shape and are arranged in a staggered mode; gaps in the horizontal direction are formed among the flow deflectors and used for air intake; and a diversion cavity is formed between the flow deflectors and the front surface of the primary filter element.