Abstract: A set of new theory of foundation design is introduced based on in-situ plate loading tests. The calculation of foundation settlement and the reasonable determination of bearing capacity in foundation design are the classic problems in soil mechanics. Although the modern soil mechanics theories have developed soil constitutive models and numerical methods to solve complex problems such as nonlinearities, the actual engineering design around the world is still using the traditional semi-theoretical and semi-empirical method. It is a problem that has not been solved well since the foundation of soil mechanics theory for nearly a hundred years. What is the root cause of the problem? How to solve it? It is believed that for the structured hard soils, the traditional theories are based on the indoor soil sample tests to obtain parameters. Due to the influences of sampling disturbances, the parameters obtained in this way cannot reflect the characteristics of the in-situ soils, so that the calculated results based on such parameters are not consistent with actual conditions and the results vary greatly. In order to solve this problem, the model for calculating the tangent modulus method is established based on the in-situ plate loading test curve, and the three soil parameters of the model are inversely calculated: initial tangent modulus $E_{\text{t0}}$, cohesion $c$ and internal friction angle$\phi$. The proposed method requires few parameters and has clear physical meaning. The parameters are derived from the in-situ tests, avoiding the influences of sampling disturbance, and the accuracy is reliable. It can be used to calculate the whole process from the nonlinearity of foundation settlement to failure. For the bearing capacity of foundation, the relation curve of the pressure and settlement (p-s curve) of the actual foundation is calculated by the tangent modulus method. According to the p-s curve, a method to determine the most suitable bearing capacity of foundation based on the principles of dual-control of strength and deformation realizes the deformation control design. At the same time, this method can solve the problem of the size effect of the bearing capacity directly determined by the plate loading test curve in the past. For the settlement calculation of soft soil foundation, on the basis of the Duncan-Chang model, a practical method for calculating the nonlinear settlement is established using the e-p curve of the compression tests. And a method for calculating e-p curve with compressive modulus $E_{\text{s}1-2}$is established, so that only the compressive modulus $E_{\text{s}1-2}$can be used to calculate the nonlinear settlement. Since the compressive modulus $E_{\text{s}1-2}$of general saturated soft soils is about 2~ 4 MPa, the range of change is small, the parameters are simple and reliable, and the proposed method is easy to apply. This research provides a new idea for solving the century-old problems of soil mechanics and a more scientific new method for modern foundation design, which is worthy of further development and improvement.