Conical deep-sea portholes (CDSP) from acrylic are widely used in underwater equipment. Due to the specific properties of acrylic, operating conditions and the stress-strain state of CDSP, it is difficult to evaluate service life and choose optimal geometric characteristics. The experimental studies and numerical calculations of the stress-strain state of CDSP from acrylic under various levels of hydrostatic pressure were made to develop an optimal design method of CDSP considering operating conditions. CDSP from acrylic were exposed to long-term (up to 7 days) and cyclic (up to 2000 cycles) hydrostatic pressure of 40, 60, 70 and 80 MPa. The deformation of the CDSP was periodically determined at no less than 20 experimental points with the help of displacement sensors. Experimental dependences of the CDSP displacements were obtained under long-term and cyclic exposure to hydrostatic pressure. According to the method of stress-time analogy, a forecast of the CDSP deformability for 10 years is made. The series of CDSP calculations with cone angles of 60–150° and relative thicknesses of 0.35–0.60 using the finite-element method was made. The calculations obtained by the finite-element method coincide with the experimental results, which demonstrated the adequacy of the adopted CDSP design scheme – continuous sliding of the translucent element with friction along the supporting surface of the porthole body. It is shown that an increase in the cone angle reduces the deflection of the CDSP, and equivalent stresses are minimal at cone angles of 75–105°. The synthesis of stress-strain state calculations by the finite-element method in aggregate with the prediction based on short-term tests allows selecting the optimal geometric characteristics of CDSP, depending on the operating conditions