A tensile testing fixture and a tensile testing device with improved specimen tensile stress accuracy are provided. The fixture includes a base, an adjusting mechanism disposed on the base, a floating mechanism disposed on and connected to the adjusting mechanism, a lower head disposed on and connected to the floating mechanism, and an upper head disposed above the lower head. The floating mechanism is configured to transmit a pulling force of the lower head to the adjusting mechanism. The adjusting mechanism is configured to adjust a position of the floating mechanism in a horizontal direction under the pulling force. The floating mechanism is further configured to adjust a position of the lower head in the horizontal direction, causing the lower head and the upper head to be coaxial in a vertical direction.