We give a simplified method to generate two types of zero-norm states in the old covariant first quantized (OCFQ) spectrum of open bosonic string. Zero-norm states up to the fourth massive level and general formulas of some zero-norm tensor states at arbitrary mass levels are calculated. On-shell Ward identities generated by zero-norm states and the factorization property of stringy vertex operators can then be used to argue that the string-tree scattering amplitudes of the degenerate lower spin propagating states are fixed by those of higher spin propagating states at each fixed mass level. This decoupling phenomenon is, in contrast to Gross's high-energy symmetries, valid to all energies. As examples, we explicitly demonstrate this stringy phenomenon up to the fourth massive level (spin-five), which justifies the calculation of two other previous approaches based on the massive worldsheet sigma-model and Witten's string field theory (WSFT). §1. IntroductionThe theory of string, as a consistent quantum theory, has no free parameter and an infinite number of states. It is thus conceivable that there exists huge hidden symmetry group which is responsible for the ultraviolet finiteness of the theory. In fact, it was conjectured by Gross 1) more than a decade ago that an infinite broken gauge symmetries get restored at energy much higher than the Planck energy. Moreover, he conjectured that, for the closed string, there existed an infinite number of linear relations among the scattering amplitudes of different string states that are valid order by order and are of the identical form in string perturbation theory as α goes to infinity. As a result, the scattering amplitudes of all string states can be expressed in terms of, say, the dilaton amplitudes. A similar result was presented in Ref.2) for the open string case.Soon after, it was discovered that 3) the equations of motion for massive background fields of the degenerate positive-norm propagating states can be expressed in terms of those of higher spin propagating states at each fixed mass level. This decoupling phenomenon was argued to be arisen from the existence of two types of zero-norm states with the same Young representations as those of the degenerate positive-norm states in the OCFQ spectrum. This was demonstrated by using massive worldsheet sigma-model approach in the lowest order weak field approximation but valid to all orders in α , and thus was, in contrast to Gross's result, valid to all energies. To compare with the usual sigma-model loop (α ) approximation, this result was argued to be a sigma-model n+1 loop result for the n-th massive level * )