The dependence of the solar cycle duration, T, on the 3 years averaged module of the large-scale sunspots magnetic fields (30-60 arcsec), B sp index, was investigated on the base of about 10,000 visual observations conducted during last eight (16-23) cycles. It was found that the duration T of the investigated cycles linearly depends on the index B sp of the magnetic fields observed during 3 years on decline phase of the solar cycle (second, third and fourth years after solar maximum T max ). Namely, the duration of the cycles T was varied between 9,5 and 12,5 years, when the magnetic index B sp was ranged from 2450 to 2600 G. An explanation for this dependence is proposed within the framework of non-linear αΩ- dynamo model. We found the following equation for the dependence of solar dynamo-period on magnetic index: T ≈ B sp 3/2. Therefore, the large observed index B sp , the longer calculated period T.