This invention provides a heat treatment method of thermoplastic film, a thermoplastic film and a manufacturing method thereof. In the heat treatment of the thermoplastic film, the thermoplastic film is extended at the horizontal direction and is conveyed in a heat treatment region (46) under the condition that the ratio of the conveying speed V1 of the film at the entrance side and the conveyingspeed V2 of the film at the exit side is 0.6 to 0.999, and the heat treatment is carried out under the condition that the vitrification conversion temperature is (Tg-20) degrees Celsius to (Tg+50) degrees Celsius; the heat treatment region is configured by means of according to that the ratio of G/D of the roller jointing length D and the length G between the rollers is 0.01 to 0.9. The feature of the thermoplastic is that: the ratio of Rth/Re is more than 0.5 and less than 1; the range of Rth/Re of the measured delay of the thickness direction at the width direction and the delay in the plane is 0.01 to 0.1; and the heat size change under the condition of 80 degrees Celsius and in 200 h is 0.001% to 0.3%.