An electric chain saw comprising a casing as well as a motor and the brake device thereof installed in the casing; wherein the brake device includes the following parts: a brake drum sleeved on and revolving together with a motor shaft; and a brake strap sleeved on outer edge of the brake drum, with one end connected with a first brake unit and the other end with a second brake unit. the casing is provided inside with a main power switch serially connected with the motor; the first brake unit includes a first coupling, a first elastic member, a button and a driving member; the first coupling is fixed on the brake strap; the button and the driving member are connected with the casing via a pivot and a pivot, respectively; the button, opposite at one end to a contact on the main power switch, is provided at the other end with a molded main tooth section; the driving member, opposite at one end to one end of the first coupling, is provided at the other end with a molded secondary tooth section engaged with the main tooth section; and the first elastic member, located between the other end of the first coupling and the casing, provides an elastic force that drives the first coupling to move towards one side to tighten the brake strap and wherein the casing is provided inside with a first passage that guides the first coupling to perform rectilinear motion, and wherein the first elastic member is a spring, through which goes one end of the brake strap before being fixed on the first coupling via a first fixing pin; the first passage has an end wall on both sides; and the spring is propped at one end against the first coupling and at the other end against the end wall on one side of the first passage, characterized in that the first coupling is provided at one end with a first shrinkage pool into which the spring can be propped and at the other end with a molded convex section; and this convex section, extending out of a pilot hole on the end wall on the other side of the first passage, is opposite to an extension handle molded on the driving member. a brake drum sleeved on and revolving together with a motor shaft; and a brake strap sleeved on outer edge of the brake drum, with one end connected with a first brake unit and the other end with a second brake unit. the casing is provided inside with a main power switch serially connected with the motor; the first brake unit includes a first coupling, a first elastic member, a button and a driving member; the first coupling is fixed on the brake strap; the button and the driving member are connected with the casing via a pivot and a pivot, respectively; the button, opposite at one end to a contact on the main power switch, is provided at the other end with a molded main tooth section; the driving member, opposite at one end to one end of the first coupling, is provided at the other end with a molded secondary tooth section engaged with the main tooth section; and the first elastic member, located between the other end of the first coupling and the casing, provides an elastic force that drives the first coupling to move towards one side to tighten the brake strap and wherein the casing is provided inside with a first passage that guides the first coupling to perform rectilinear motion, and wherein the first elastic member is a spring, through which goes one end of the brake strap before being fixed on the first coupling via a first fixing pin; the first passage has an end wall on both sides; and the spring is propped at one end against the first coupling and at the other end against the end wall on one side of the first passage, characterized in that the first coupling is provided at one end with a first shrinkage pool into which the spring can be propped and at the other end with a molded convex section; and this convex section, extending out of a pilot hole on the end wall on the other side of the first passage, is opposite to an extension handle molded on the driving member.