Analytical solution of tensile stress of square specimen under splitting load was obtained through Fourier Series Solution of plane stress problems. According to the stress distribution of the un-cracked square specimen, the calculation formula of Mode I fracture toughness of cracked straight through the square specimen, CSTSS, under splitting load was obtained, and the shape function was corrected by the finite element interaction integration method. In order to verify the correctness of the

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... rmula, a CSTSS splitting test was performed. The test results indicated that the stress distribution on the axis of symmetry is consistent with the theoretical results, and the minimum tensile stress occurs at the center point of the un-cracked specimens. The Mode I fracture toughness of CSTSS, KIC, was stabilized in relative range and the average value of KIC was 0.140MPa×m1/2. The three-point-bend experiment was introduced as a comparative test and the average value of KIC was 0.158MPa×m1/2 which was slightly larger than that of CSTSS. The test results indicated that both of the two tests could be reliable and the CSTSS under splitting load could be a potential method to determine the Mode I fracture toughness of rock.