The invention relates to a method and device for measuring electroencephalo of a human body having mental pressure and for relaxation training. The device provided by the invention comprises an electroencephalo collecting electrode, a preprocessing unit, an analog-to-digital conversion unit, a central control unit and earphones. The method provided by the invention comprises the steps of: firstly, collecting initial electroencephalo data; secondly, calculating SI(0); thirdly, providing guiding respiratory parameters; fourthly, recording the respiratory parameters; fifthly, collecting electroencephalo data; sixthly, calculating SI; seventhly, judging whether SI is decreased or not; eighthly, keeping M(i) constant; and ninthly, adjusting the respiratory parameters. The method provided by the invention can better reflect the mental stress of a user and have high degree of accuracy and good real-time performance; the device provided by the invention has fewer electrodes and is convenient for installation; and a collection circuit and other hardware structure parts are simple, and the manufacturing cost is very low. According to the method and device provided by the invention, a noninvasive method for quantificationally assessing mental stress or mental pressure and a mental pressure relaxation training device based on the method are realized, which have certain easing function on mental pressure of the human body.