A follow-up fixed focus system, comprising: a carrying platform; a sample plate fixed on the carrying platform; a support frame vertically connected to the carrying platform; an observation device slidably arranged on the support frame, wherein the observation device comprises a camera or an eyepiece, an objective lens and a lens barrel part; and a fixed focus device having one end fixedly connected to the observation device and another end in contact with the sample plate, wherein the fixed focus device maintains a vertical distance between the sample plate and the objective lens to be unchanged, characterized in that the fixed focus device comprises a support rod and a light-transmittable component, and the support rod has one end fixedly connected to the observation device and another end fixedly connected to the light-transmittable component, and a space between the objective lens and the light-transmittable component is hollow, the follow-up fixed focus system further comprises a mechanical elastic device connected to the support frame and the observation device or connected to the support frame and the fixed focus device, wherein the mechanical elastic device supports the observation device and the fixed focus device, wherein in the case that the observation device is upright, the mechanical elastic device comprises: a position-limiting component arranged on the support frame; a connecting rod running through the position-limiting component, wherein the connecting rod has one end fixedly connected to the observation device or the fixed focus device and another end provided with a projection; and a spring sleeved on the connecting rod and stuck between the position-limiting component and the projection, or wherein in the case that the observation device is upside down, the mechanical elastic device comprises: a position-limiting component arranged on the support frame; a connecting rod running through the position-limiting component, wherein the connecting rod has one end fixedly connected to the observation device or the fixed focus device and another end provided with a projection; and a spring sleeved on the connecting rod and stuck between the position-limiting component and the observation device. a carrying platform; a sample plate fixed on the carrying platform; a support frame vertically connected to the carrying platform; an observation device slidably arranged on the support frame, wherein the observation device comprises a camera or an eyepiece, an objective lens and a lens barrel part; and a fixed focus device having one end fixedly connected to the observation device and another end in contact with the sample plate, wherein the fixed focus device maintains a vertical distance between the sample plate and the objective lens to be unchanged, characterized in that the fixed focus device comprises a support rod and a light-transmittable component, and the support rod has one end fixedly connected to the observation device and another end fixedly connected to the light-transmittable component, and a space between the objective lens and the light-transmittable component is hollow, the follow-up fixed focus system further comprises a mechanical elastic device connected to the support frame and the observation device or connected to the support frame and the fixed focus device, wherein the mechanical elastic device supports the observation device and the fixed focus device, wherein in the case that the observation device is upright, the mechanical elastic device comprises: a position-limiting component arranged on the support frame; a connecting rod running through the position-limiting component, wherein the connecting rod has one end fixedly connected to the observation device or the fixed focus device and another end provided with a projection; and a spring sleeved on the connecting rod and stuck between the position-limiting component and the projection, or wherein in the case that the observation device is upside down, the mechanical elastic device comprises: a position-limiting component arranged on the support frame; a connecting rod running through the position-limiting component, wherein the connecting rod has one end fixedly connected to the observation device or the fixed focus device and another end provided with a projection; and a spring sleeved on the connecting rod and stuck between the position-limiting component and the observation device. a position-limiting component arranged on the support frame; a connecting rod running through the position-limiting component, wherein the connecting rod has one end fixedly connected to the observation device or the fixed focus device and another end provided with a projection; and a spring sleeved on the connecting rod and stuck between the position-limiting component and the projection, or wherein in the case that the observation device is upside down, the mechanical elastic device comprises: a position-limiting component arranged on the support frame; a connecting rod running through the position-limiting component, wherein the connecting rod has one end fixedly connected to the observation device or the fixed focus device and another end provided with a projection; and a spring sleeved on the connecting rod and stuck between the position-limiting component and the observation device.